- Email: [email protected]
Lignocellulosic conversion into value-added products: A review
Journal Pre-proof Lignocellulosic conversion into value-added products: A review Dibyajyoti Haldar, Mihir Kumar Purkait
PII:
S1359-5113(19)31090-6
DOI:
https://doi.org/10.1016/j.procbio.2019.10.001
Reference:
PRBI 11789
To appear in:
Process Biochemistry
Received Date:
17 July 2019
Revised Date:
14 September 2019
Accepted Date:
3 October 2019
Please cite this article as: Haldar D, Purkait MK, Lignocellulosic conversion into value-added products: A review, Process Biochemistry (2019), doi: https://doi.org/10.1016/j.procbio.2019.10.001
This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier.
Lignocellulosic conversion into value-added products: A review Dibyajyoti Haldar*, Mihir Kumar Purkait* Centre for the Environment, Indian Institute of Technology Guwahati, Assam-781039, India
---------------------------------------*
ro of
Corresponding authors Dr. Mihir Kumar Purkait ([email protected]), Professor, Department of Chemical Engineering, IIT Guwahati, India. Dr. Dibyajyoti Haldar ([email protected]), IPDF, Centre for the Environment, IIT Guwahati, India.
Highlights
re
-p
Involvement of several global agencies to strengthen biofuel production. Insight into the progress in various pretreatment technologies. Aspect of cellulase for an effective saccharification process. Genetic and metabolic engineering towards an enhanced biofuel production. Fate of lignocellulosic conversion into the derivatives of cellulose and lignin.
Jo
ur
na
lP
1
Abstract In the present context of energy crisis, exploration of lignocellulosic biomass has emerged as potential substitute to maintain environmental sustainability. However, the conversion of biomass into value-added products still faces challenges to find a suitable unit operation. The stubborn dependency on the cost intensive enzymatic system, limits an effective saccharification to hydrolyze the biomass. India has been one of the top most agriculturally enriched countries with broad scope of utilizing waste residue that remains as an unused
ro of
biomass at harvested locations. Hence, in the present script, an overview on the latest R&D initiatives taken by Government of India are briefed to highlight the projects based on biofuels and in addition, global scenario of biofuel production is comprehensively discussed.
-p
Further, critical analysis on the advancement of different pretreatment operations are highlighted through latest inventions. Thereafter, biochemistry of cellulase enzyme with
re
essential factors were explored to understand the mechanistic interactions involved during
lP
saccharification of biomass. An insight on the latest accomplishments of various fermentation process provides an in depth understanding of metabolic engineering based on the genetic
na
studies of fermentative microorganisms. Finally, the article is concluded with brief discussions on fate of the derivatives obtained from macromolecules such as cellulose and
ur
lignin.
Key words: Lignocellulose; Biomass conversion; Value-added products; Hydrolysis;
Jo
Fermentation.
2
List of abbreviations R&D: Research and Development SHF: Separate Hydrolysis and Fermentation SSF: Simultaneous Saccharification and Fermentation ABE: Acetone Butanol Ethanol CBP: Consolidated Bioprocessing HMF: Hydroxymethyl Furfural
ro of
CO2: Carbon-Di-Oxide DBT: Department of Biotechnology NREL: National Renewable Energy Laboratory
-p
IOCL: Indian Oil Corporation Limited ICT: Institute of Chemical Technology
re
BPCL: Bharat Petroleum Corporation Limited
lP
MOP: Ministry of Power
DST: Department of Science and Technology
na
MNRE: Ministry of New and Renewable Energy
CSIR: Council of Scientific and Industrial Research
ur
MOPNG: Ministry of Petroleum and Natural Gas DP: Degree of Polymerization
Jo
CBD: Cellulose Binding Domain CD: Catalytic Domain CBH: Cellobiohydrolase NADH: Nicotinamide Adenine Dinucleotide Hydrogen
3
Contents 1.
Jo
ur
5.
na
lP
4.
re
3.
-p
ro of
2.
Introduction 1.1. Present scenario of biofuel production in India 1.2. Scenario on global production of biofuel Conventional pretreatment methods 2.1. Pretreatment with aid of physical methods 2.1.1. Pretreatment using ball milling 2.1.2. Pretreatment using extrusion 2.1.3. Pretreatment with the aid of irradiation 2.2. Pretreatment with aid of chemicals 2.2.1. Pretreatment of lignocellulosic biomass using acid 2.2.2. Pretreatment of lignocellulosic biomass using alkali 2.2.3. Pretreatment of lignocellulosic biomass using ionic liquid 2.2.4. Pretreatment of lignocellulosic biomass using organic solvents 2.3. Physico-chemical pretreatment 2.3.1. Steam explosion pretreatment 2.3.2. Liquid hot water pretreatment 2.3.3. Wet oxidation pretreatment 2.4. Biological pretreatment 2.5. An insight on latest and advanced emerging pretreatment technologies Strategies towards improved enzymatic hydrolysis of biomass 3.1. Factors influence the process of enzymatic hydrolysis 3.2. Cellulase enzyme: Biochemistry and mechanistic interactions 3.3. Efforts to overcome the criticalities involved with cellulase Strategies involved in the fermentation of sugars into biofuel 4.1. Consolidated bioprocess (CBP) involved in fermentation 4.2. Simultaneous saccharification and co fermentation 4.3. Simultaneous pretreatment and saccharification 4.4. Exploration of metabolic engineering towards improved biofuel production 4.4.1. Engineered saccharomyces cerevisiae for bioethanol production 4.4.2. Engineered Zymomonas mobilis for bioethanol production 4.4.3. Genetic engineering in Clostridium acetobutylicum for biobutanol production Conversion of biomass into the derivatives of cellulose and lignin 5.1. Derivatives of cellulose 5.1.1. Formic acid 5.1.2. Acetic acid 5.1.3. Levulinic acid 5.1.4. Sugar alcohol 5.1.5. Hydroxymethyl furfural (HMF) 5.2. Derivatives of lignin 5.2.1. Phenol 5.2.2. Vanillin 5.2.3. Carboxylic acids 5.3. Detoxification of inhibitors Future perspective Conclusions
6. 7.
4
1. Introduction In order to meet the growing demand of renewable energy, adequate sources of fossil fuels are inadequately utilized as the most vulnerable resource of this globe [1-3]. Besides, rapid urbanization including global industrialization is equally anticipated with fast exploitation of the fossil fuels. Therefore, alternative search towards the production of green fuels using renewable feedstock like lignocellulosic biomass has been a potential choice as the biomass does not necessarily compete with conventional food crops. Further, the process involved in
ro of
the production of biofuel (e.g., bioethanol, biobutanol) from biomass has been of low toxicity and cost effective. Hence, with regard to an industrial acceptability, advancements in the process of pretreatment, saccharification, and fermentation has been encouraging as top
-p
research endeavor.
Pretreatment has been an essential unit operation in order to beak the outermost seal of lignin
re
and subsequent exposure of carbohydrates (cellulose and hemicellulose) towards the process
lP
of hydrolysis [4-6]. Primarily, the process of pretreatment largely dictates the overall cost of the technology as it directly deals with heterogeneous sources of lignocellulosic biomass [7,
na
8]. Therefore, a number of different pretreatment methods have always emerged as motivation to architect an optimum reaction condition for each of the individual feedstock
ur
materials. With regard to the nature of the reaction, pretreatment can be subcategorized into different classes as physical, chemical, physico-chemical and biological processes [9-11].
Jo
Physical pretreatment attributes towards size reduction of biomass particle with an increase in an effective surface area manifesting a reduction in degree of polymerization and physicochemical pretreatment involves chemical addition along with combination of physical processes [12, 13]. Among different physico-chemical pretreatment methods, steam explosion, liquid hot water and wet oxidation are mostly studied [14]. Chemical pretreatment involves an incorporation of different chemicals such as acid, alkali, organic solvents and 5
liquid mixture of ions in the reaction mixture. Finally, biological pretreatment uses a number of several microorganisms that excrete enzymes to deconstruct the rigid structure of biomass [15]. Each of the pretreatment methods has their own limitations. An attempt towards the robust deconstruction of biomass materials through pretreatment drives towards advancement in the process based on the existing procedures. Following pretreatments, hydrolysis using enzyme assists in the production of monosugars from biomass [16]. A number of several attempts are made to ease out the high cost
ro of
associated with enzyme [17]. Though enzymatic hydrolysis is not resulted into the formation of inhibitors but the inhibitor free formation of sugars partially inhibits action of the enzyme due to product inhibition during the process of separate hydrolysis and fermentation (SHF).
-p
On the other side, simultaneous saccharification and fermentation is conducted using a single reactor to overcome the problem of product inhibition and improved the product yield within
re
a short span of time [18-20]. Fig. 1 depicts the overall process involved in the production of
lP
biofuel from biomass which represents the schematic of each of the individual steps of pretreatment, hydrolysis and fermentation process. Lignocellulosic biomass has been the reservoir of carbon in different polymeric forms of
na
cellulose, hemicellulose and lignin [21-23]. Therefore, the biomass is subjected to be utilized as a potent sustainable bioresource, once it is transformed economically into value-added
ur
products. The technologies involved in the bioconversion process primarily focuses on sole
Jo
production of the specific valuable product [24-26]. Formation of selective derivatives generated from cellulose and lignin still be a promising approach in order to get rid of additional cost during separation in downstream processing. Exhaustive explanations on various pretreatment techniques are readily available in the literatures to evaluate the performance of various uniform or combined chemical and physicochemical operations for disintegration of biomass. In view of that, Sarkar et al.
6
reviewed different pretreatment methods for the production of bioethanol from agricultural residues with mere highlights on each of the pretreatment methods [27]. Moreover, Chen et al. provides an exhaustive narration on various pretreatment methods for lignocellulosic conversion into high value chemicals without detailing the formation of any value-added chemicals as a result of pretreatment conversion [28]. Further, Alvira et al. highlights on the key factors influence the process of the pretreatment technologies for an efficient conversion into bioethanol, based on enzymatic hydrolysis [29]. However, none of the articles on
ro of
pretreatment process provides any in-depth discussion on any of the particular unit operation. Whereas, in our present article at the very beginning of pretreatment section an emphasize was given on various aspects of mechanical pretreatments with an aspiration of exploring the
-p
promising ability of ball milling and extrusion towards an effective deconstruction of biomass. Additionally, recent advancements in each of the specific unit operations were
re
explored to determine the progress specifically in the sector of pretreatment. Further, a
lP
number of different approaches were attempted to hydrolyze the biomass using a variety of cellulolytic enzymes. In view of that, a clear mechanistic insight on the biochemistry of cellulase enzyme will facilitate to select the type of the target specific enzyme required for a
na
particular biomass. Following hydrolysis, the processed biomass is subjected to fermentation process for value-addition. Further, instead of mostly reported conventional approaches of
ur
SHF and SSF, combinations of different fermentations techniques with a better genetic and
Jo
metabolic understanding of microorganism seems beneficial to intensify the process with respect to biofuel production from biomass. In view of an improved production, various routes of fermentation demands an in-depth assessment on several factors. Yang et al. observed various challenge and prospects involved with solid-state anaerobic digestion for an efficient conversion of lignocellulosic biomass [30]. However, the promising aspect of exploring metabolic route of several fermentative microorganisms could have been beneficial
7
for process scale up. Thereafter, Kumar et al. reviewed new insight on the development in biobutanol production without considering the essence of bioethanol as one of the major fermentative product of lignocellulosic biomass [31]. Thereafter, an explicit discussion on the transformation of biomass into the selective derivatives of cellulose and lignin will largely cover the entire aspect of lignocellulosic conversion into value-added products [32-34]. The present review discusses on the latest advancements in the process of pretreatment, saccharification and fermentation of lignocellulosic biomass. In view of that, the global
ro of
scenario of biofuel production is discussed based on the initiatives taken by different Governmental agencies. Further, critical analysis on various conversion process during pretreatment are conducted and supported by latest accomplishments. With respect to
-p
mitigate the bottlenecks involved in the process of hydrolysis and fermentation, an insight on the efficacy of microbial community is explored for an improved production of bioethanol
re
and biobutanol as biofuel. Finally, various routes for the conversion of cellulose and lignin
lP
into their derivatives manifests an entire aspect of exploring renewable lignocellulosic biomass as an indispensable option for environmental sustainability. 1.1. Present scenario of biofuel production in India
na
With the present stand in 2019, India has been the fastest economy of the world. Inspite of that, the country still urges to develop sustainable technology in the sector of biofuel to
ur
mitigate the sole dependency on petroleum based fuels. With the aspirations to strengthen
Jo
country’s energy security, planning commission of Government of India released a report in the year of 2003 and mandates regarding 5% ethanol blend with conventional fuel. Further, the next commission increased the mandate to 10% of bioethanol blend for the period from 2007 to next five years. In order to mitigate such long term bottlenecks, Government of India has already issued a recommendation under the agenda of ‘National policy on biofuels’ [35]. The proposal was aimed at minimum level of biofuel makes readily available to meet the
8
energy demand of the country with an aspiration of blending conventional petroleum derived fuel with 20% biofuel including biodiesel and bioethanol by the end of 2030. With regard to international context and national circumstances, the policy is dedicated towards constructing an interdepartmental mechanism to supervise on the developments aimed at more efficient technologies for biofuel production based on indigenous feedstock. In the late 2011, Department of Biotechnology (DBT) under the Ministry of Science and Technology, Government of India ventured a collaborative research work with Indian Oil Corporation
ro of
Limited (IOCL) to set up a steam explosion plant at Centre for Advanced Bioenergy Research in Faridabad, New Delhi. The Centre was fully operative in June 2012 when the pilot plant was installed and commissioned with a collaborative agreement with National
-p
Renewable Energy Laboratory (NREL), USA. The steam explosion pretreatment plant facilitates a through put of 5 KG of dry biomass per hour and subsequent bench scale batch
re
facility for saccharification and fermentation. Recently, as a part of Biofuel mission by
lP
Government of India, Ministry of Science and Technology supported DBT-ICT has started full-fledged demonstration plant for the production of ethanol from lignocellulosic biomass at Centre for energy biosciences at ICT Mumbai with a capacity of 10 tons of biomass/day in
na
April 2016. Likewise, with the prior approval from Ministry of Petroleum and Natural Gas, Bharat Petroleum Corporation Limited (BPCL), is all set to architect India’s first 2G refinery
ur
producing bioethanol at the Bargarh district in Odisha. BPCL is well dedicated to invest
Jo
13,95,76,100 USD to produce 3 crore litre of bioethanol annually with a consumption of around two lakh metric tons of rice straw available locally. Apart from a major thrust, accorded to Research and Development sector, Government of India is to prioritize major innovation based on local feedstock and indigenous technology to attributes towards set up of more pilot scale plants in India. Fig. 2 depicts budgetary allocations implemented through different R&D agencies of Government of India in the recent years, it can be clearly seen that
9
allocations has been increased with the years from 2015 to 2018 manifests Governments’ attempt to boost major research areas in the sector of clean energy through various R&D programme. Maximum fund has been disbursed to MOP followed by DST, MNRE, CSIR, MOPNG and DBT. Table 1 highlights on recent R&D projects on biofuel sanctioned by different agencies of Government of India. From the table it can be comprehended that, around 1,68,936 USD is already sanctioned for various biofuel based R&D projects. 1.2. Scenario on global production of biofuel
ro of
The global demand of biomass based liquid fuel is increasing due to the overrated exploitation of fossil fuels with its subsequent detrimental effect on earth and atmosphere. During the period of 2008 to 2017, the atmospheric CO2 concentration has escalated high into
-p
a level of ~2.3 ppm/year [36]. Fossil fuels attributes towards 80% of primary global energy consumption and out of which ~60% of energy is consumed by the transport sector [37]. As a
re
consequence, of such anthropogenic events, the mean global surface temperature is increased
lP
by 0.85 °C that eventually manifests towards an increased sea level after rapid reduction for ice and snow. With respect to the increase in detrimental gases in atmosphere, EU adopted Kyoto Protocol in the year of 1998, with an aspiration to reduce the emission of greenhouse
na
gasses at or below of 8% during 2008 to 2012. In order to achieve such directives, a number of initiatives were improvised for the better utilization of energy from renewable based
ur
sources. In 2009, renewable energy directives of EU mandated to each of the state’s partner
Jo
towards specific target of 20% of renewable energy consumption along with 20% reduction in greenhouse emissions and 20% increase in energy efficiency by the end of 2020. Since the implementation of the directives, an encouraging 12.5% of total energy consumption is obtained from renewable sources in different states of EU. By 2020, a maximum of 7% first generation biofuel was aimed to introduce in the transport sector. With the revised renewable energy directives of 2016, an agenda was proposed to ensure of at least 27% renewable
10
energy consumption in EU by 2030. Apart from EU, USA has been the world largest producer of biofuel in 2017 with a sole production 16 billion gallon of bioethanol. Since its emergence to support the progress of biofuel as an alternative source of energy over gasoline based fuels, the Government of USA has been keen to provide necessary incentives through various regulations. As a part of implementing the regulation to explore the emergence of biofuel, The Government of USA started with the Energy Tax Act of 1978 [38]. As a consequence, biofuel producers are decided to exclude from paying Federal Gasoline Excise
ro of
Tax with an expectation of producing 10% ethanol blended gasoline. Further, Energy Independence and Security Act of 2007 ensured uninterrupted growth in bioethanol production with futuristic mandates on biofuel usage in USA [39]. Primarily, corn is
-p
considered as main feedstock for bioethanol production in USA with a recorded annual production of 15.1 billion bushels in the year of 2016-17. Following US, Brazil is another
re
leading producer of lignocellulosic ethanol in the world. Brazil is reported to produce 30.7
lP
billion liters of ethanol against the total domestic demand of 28.7 liters in 2018. Utilization of sugarcane as lignocellulosic feedstock attributes towards 61% contribution in bioethanol production of the country. On the contrary, compared to 2017, an increase of 58% was
na
observed in the year of 2018 when ethanol was produced using corn as lignocellulosic feedstock. In an addition, Brazil is well committed towards a mandate of 27% ethanol
ur
blending for gasoline with an aspiration of greenhouse reduction of 37%, 43% by the end of
Jo
2025 and 2030 respectively. Like sugarcane and corn in Brazil, the development of biofuels in China primarily based on an effective utilization of feedstock’s like corn, maize, wheat and rapeseed [40]. Inspite of second largest economy of the world, China’s renewable growth rate is considered insignificant, as the contribution of biofuel remains low in the sector of bioenergy. However, the country’s ambition of reducing greenhouse gas emission becomes more realistic when 82% growth in renewable energy was accounted compared to only 16%
11
growth in the consumption of fossil fuels during 2010-2015 [41]. As of present time, China has completely transformed its status from producing zero biofuel into the fourth largest producer in world. As a result, an amount of 9,770 million liters of ethanol production is forecasted in the year of 2018. Nevertheless, Chinas’ long-term aspiration directs near to fivefold escalation in ethanol consumption from the present is severely interrupted due to country’s environmental policies and strict economic regulation.
2. Conventional pretreatment methods
ro of
2.1. Pretreatment with aid of physical methods
Physical pretreatment is aimed to impose mechanical forces on biomass in terms of impact, friction, shear and compression. Most of the major mechanical forces are generated from the
-p
impact and friction within biomass particles [42]. A sieve or screen allows the control on the particle size of desired product. Based on the particle size of the desired product, cutting
re
strategies are subcategorized into crushing (meter to centimeter), milling (cm to 500 µm),
lP
micronization (cm to 100 µm), fine grinding (less than 100 µm), ultra-fine grinding (less than 30 µm) [43]. A number of different tools such as ball mill and extruder cut the biomass into
na
different fragments in order to achieve the desired particle size as the product. During ball mill operation, biomass undergoes impact and compression stress when collision occurs with
ur
ball and wall of the reactor. Due to the generated stress within the ball mill, deconstruction of biomass attributes towards more accessible area readily available for subsequent hydrolysis
Jo
process.
12
2.1.1. Pretreatment using ball milling Primarily, ball milling is conducted with biomass and ceramic balls together in a mill at a specific rotation represented as rpm. The collision between biomass particle with the balls at a fixed rotation leads to break the crosslinks in between cellulosic chain that attributes to a disruptive structure resulting in more porous formation within the surface of biomass [44]. In view of that, ball mill generally uses ball made up of metal or ceramic to grind up the biomass particles. Based on the movement of the balls within the mill, ball mills are
ro of
classified into conventional, attrition and planetary mill. Conventional mill utilizes the natural force of gravity on the biomass particles, planetary mills generated artificial force from centripetal movement inside the mill and attrition mills rotates the rotor to stir the grinding
-p
balls. Due to low efficiency, conventional ball mills are mostly used in a combination with chemical pretreatment. In case of planetary mill, milling resulted from the movement of
re
rotating bowl and the balls whereas, milling is generated from the stirring action of rotating
lP
impeller with the arms for attrition mills. Inspite of high energy demand, ball mills possess a promising approach to reduce particle size and degree of crystallinity of biomass. Lu et al. investigated that the change in particle size with milling time and at the initial phase of
na
milling an exponential decrease in the median diameter of cellulose particles attributes towards a maximum 47% of decrease followed by an insignificant change in particle size at
ur
further increase in milling time. Also, after 60 min of milling period, specific surface area
Jo
was observed to increase to a maximum of 2.25 m2/g compared to a surface area of 1.37 m2/g of native cellulose [45]. Further, the ball mill facilitates ~23% reduction in degree of polymerization (DP) of biomass after 40 min of continuous rotation. Thus, the decrease in DP is due to the cleavage of β-(1-4) glycosidic linkages in linear cellulosic chain manifests the severity of ball milling pretreatment in disrupting polymeric nature of cellulose. Other than rotational speed, water content and reaction temperature are the critical factors in determining
13
the effect ball milling pretreatment on biomass. Gu et al. observed fast grinding of corn stover when milling was conducted without of using any water in the reaction mixture. A maximum 67.0% (w/w) of glucose was produced when corn stover was milled in the absence of water at 80 °C for 30 min. [46]. As water was studied to show significant effect on biomass hydrolysis during ball milling technique. Hence, in an another study, the effect of ball milling technique was intensified using combined pretreatment of biomass and a mixture of solvents (water, ethanol, sulphuric acid and hydrogen peroxide) at varying temperatures in
ro of
the range of 100 to 150 °C with an interval of 10 °C. It was noticed that, xylose was the only sugars obtained from corn straw at lowest temperature of 100 °C (without ball milled). However, glucose was released only when ball milled sample was reacted at or above 120 °C
-p
[47]. Thereafter, an insignificant increase in sugars yield was observed with increase in temperature for ball milled sample. Table 2 represents the effect of ball mill pretreatment on
re
different lignocellulosic biomass. From the table it can be observed that, combined treatment
lP
primarily preceded by ball milling resulted into more severe disruption in the structure of biomass. Beside a number of advantages, high energy input during the process has been a
na
major challenge towards the practical applicability of ball mill pretreatment, which demands to address the issues dependent on; (i) rotation speed of the motor, (ii) capacity of ball mill,
ur
(iii) structure and properties of native biomass and (iv) desired particle size. Among different advantages, reduction in cellulose crystallinity and degree of polymerization of biomass are
Jo
most important outcome whereas, high power input during ball milling slows down the practical applicability of the process. 2.1.2. Pretreatment using extrusion After ball milling, extrusion has been another promising pretreatment method which involves heating, mixing and shearing of biomass that primarily brings about the physical and chemical alterations attributes to significant exposure of cellulose towards subsequent 14
hydrolysis process. The sole efficiency of this process to operate at high solid loading advances its effect when combined with other chemical methods. During the process of extrusion, moderate operating conditions within the reactor (known as extruder) facilitates less formation of inhibitory by products. A number of research studies on extrusion pretreatment hypothesized the fact that, the variation in sugar yield largely depends on the process condition and nature of biomass used. Table 3 summarizes different operating conditions for different biomass to improve the process of extrusion. The table shows
ro of
moisture present in the biomass, temperature of the reaction media, screw speed and screw elements are essential for an effective extrusion process attributed to substantial decrystallization and delignification of biomass. A maximum of 69.5% delignification was
Extrusion process and configuration of extruder
-p
obtained for corn stover.
re
Extrusion has been a widely acknowledged thermo-mechanical pretreatment process based on
lP
the shear force exerted by the rotating screws on the biomass within the extruder. The reactor is consisted of screws rotate at a specific rotation using motor within a barrel. Single screw extruders are constructed with single solid screw within barrel of extruder while twin screw
na
extruders are architected with multiple screw-like elements assembled over shaft. Based on the type of the screw material used, twin screw extruders are classified as co-rotating and
ur
counter-rotating extruders and the biomass is feed through the hopper section. Fig. 3 shows
Jo
the schematic of a conventional twin screw extruder. The extruder is configured with a hopper for the collection of sample biomass. Motor supplies the energy to rotate both the screws. The barrel mainly represents the reaction zone which is thermally regulated. Due to the differences in the configuration of screw elements, the operation of extruders largely differs in the way the biomass is mixed and flows through the barrel. Compared to single screw extruder, twin screw is observed to produce more intense mixing and reduction in
15
particle size thus attributes towards more rigorous changes in the physical properties of biomass. Apart from the good mixing properties, counter rotating screws are advantageous because of self-cleaning ability. Factors influence the process of extrusion Like the other pretreatment processes, extrusion also depends on a number of interrelated parametric conditions. Among all, screw profile (configuration, rotation), temperature and solid loading inside the reactor mostly influence the extrusion process.
ro of
Screw profile and speed used in extruder The change in the properties of biomass depends on the mechanical force generated due to the variations involved in the screw configurations. Apart from the mechanical forces, screw
-p
profile is also considered on a high extent to fixed up other parametric variables such as solid loading. Further, basic arrangements of single screw or twin screw extruders demand more
re
incorporation of associated elements to advance the process of mixing. Among various
lP
orientations in the configuration of screws, application of reverse kneading disk was appeared to exert maximum pressure to disintegrate the hardwood (Angiosperm trees produced hardwood biomass and seeds of this pants are mostly covered) biomass effectively [48].
na
Additionally, incorporation of reverse elements in the screw improves residence time of the biomass thus excel cellulose digestibility with reduction in crystallinity [49]. Further,
ur
residence time of the biomass highly depends on screw speed. With a fixed intake of
Jo
biomass, residence time decreases with increase in screw speed. Most of the extrusion pretreatments are confined within low rotation in screw speed to increase the residence time. In view of that, optimum screw speed of 7 Hz was obtained for twin screw extruder when wheat bran was used as lignocellulosic biomass and in the same experiment, Lamsal et al. identified optimum screw speed of only 3 Hz when soybean hulls were used [50]. Karunanithy et al. used only 1.7 Hz as optimum screw speed in single screw extruder for the
16
pretreatment of switch grass [51]. Apart from screw profile and speed, temperature of the extruder has been an another dictating factor associated with the process of extrusion. Temperature Temperature within the extruder has been an important parameter as it directly impacts on the hydrolysis of biomass. However, at high temperature possible condensation of lignin frequently limit the rate of biomass hydrolysis. Sometimes, bio char is also formed from excessive torrefaction of biomass at high temperature [49]. Further, presence of catalyst
ro of
reduces the required temperature and it was often noticed that, extrusion carried out at acidic pH necessitates high temperature [52]. Hence, low temperature has been quite imperative at low pH conditions to avoid the formation of degradation products from sugars or lignin of
-p
biomass. Liquid to solid ratio of biomass
re
The aspect ratio of liquid to solid refers the amount of liquid added into the reaction mixture
lP
which is required to maintain the flow during the process of extrusion. Addition of liquid leads to soften the polymeric cellulose and subsequently breaks the network of microfibril bound. Though, excessive addition of water need to be restricted to avoid over dilution of the
na
sample produced by extrusion pretreatment. During non-catalyzed reactions, the moisture content present within the biomass largely impact on the development of shear forces
ur
resulting in substantial effect on biomass disintegration. The formation of thin film due to
Jo
over absorption of moisture by corn stover substantially reduce the effect of wet ball mill pretreatment [53]. Limited sugars degradation and high continuous throughput are the merits of extrusion process employed for the pretreatment of biomass prior to hydrolysis and highenergy input and instrumental complexities are the subsequent limitations of this technology.
17
2.1.3. Pretreatment with the aid of irradiation The physical pretreatment processes so far discussed are milling and extrusion primarily concerned to increase the surface area of biomass at an expense of high amount of energy consumption. Therefore, an alternative of using irradiation in the form of gamma ray or electron beam has emerged as a potential replacement. On an addition, radiation technique has been well established to effectively disintegrate cellulose for most of the lignocellulosic biomass [54-56]. Further, the process of irradiation is not associated with the requirement of
ro of
high temperature and no inhibitors are normally produced in the hydrolysate as a result of the irradiation pretreatment. Though, to increase the effectivity, irradiation treatment is frequently performed in combination with chemical treatment [57, 58]. During the combined
-p
treatment, initially irradiation alters the physical structure of biomass followed by the penetration of the chemical agent into the biomass. Lee et al. irradiated kenaf core biomass
re
with 500 kGy of electron beam and following acid treatment with 3% (v/v) sulphuric acid,
lP
the biomass produced a maximum of 73.6% sugar yield after 72 h of enzymatic hydrolysis [57]. Furthermore, the effect of the sequence followed during two stage pretreatment of irradiation and chemical addition substantially improves the sugars yield. Yin et al. showed
na
that, the performance of wheat straw irradiated then swallowed with 4% (w/v) sodium hydroxide was comparatively better than chemical soaking before gamma ray irradiation.
ur
During this investigation, the ability of irradiation to reduce alkali consumption with time
Jo
was noticed when 72.7% (w/w) of reducing sugar was obtained after an hour at 100 kGy of irradiation and in the presence of 2% (w/v) of sodium hydroxide [58]. According to them, the effect of irradiation on substantial lignin removal significantly exposed the cellulosic structure towards sodium hydroxide swelling. However, in an another experiment conducted by Kapoor et al. showed that during the irradiation pretreatment of sugarcane bagasse, gamma ray merely produced any significant changes in the lignin content of pretreated
18
biomass [59]. Further, compared to raw biomass, pretreated biomass showed comparable cellulose content when treated under low dosage of irradiation. Thereafter, significant decrease in cellulose content was noticed at each of the individual increase in irradiation dosages. Further, the impacts of different irradiation methods during the pretreatment of biomass greatly varies from one biomass to another. Hence, Table 4 summarizes an assessment of irradiation pretreatment on different lignocellulosic biomass. Different parameter of the biomass like composition of cellulose, hemicellulose, lignin, crystallinity,
ro of
particle size is primarily affected due to the irradiation pretreatment. Irradiation dose has a significant role in the change of cellulose crystallinity for different lignocellulosic biomass.
2.2. Pretreatment with aid of chemicals
-p
2.2.1. Pretreatment of lignocellulosic biomass using acid
Cellulosic chains of lignocellulosic biomass are well connected through β-(1-4) glycosidic
re
bonds of repetitive glucose units. During the pretreatment of biomass using acids, hydrogen
lP
ion attacks the glycosidic oxygen and subsequently breaks the monomeric sugar molecules apart from polymeric chain. Hence, introduction of acids during the pretreatment technology
na
appeared as an effective process to covert the biomass into value-added products. In view of that, a number of inorganic acids such as sulphuric acid, hydrochloric acid, nitric acid,
ur
phosphoric acid are mostly applied on a variety of lignocellulosic biomass to further improve the production of sugars [60, 61]. During the acidic interaction, hemicellulose fraction of
Jo
biomass is partially hydrolyzed and part of the lignin is slightly solubilized [11, 62]. It has been observed that acid concentration and reaction period are the two most important factors during acid pretreatment process. During the reaction, application of diluted acid is sometimes preferred to substantially produce oligomers of xylose and glucose with a comparative production of monomeric sugars (glucose and xylose) at longer residence time. Nonetheless, longer residence time leads to degrade the cellulosic and hemicellulosic derived 19
sugars into organic acids and furans. Further, diluted sulphuric acid at a concentration of 0.05% (w/w) showed severe degradation efficiency of sugars into organic acids and furans compared to 0.01% (w/w) of sulphuric acid at a comparable reaction condition during two stage acid treatment of cassava [63]. In an another approach of fractional hydrolysis by Mishra et al., four different inorganic acids at varying concentrations were tested on kans grass biomass for maximum fermentable sugar recovery with minimum toxicity. Among the acids, HNO3 recovered a highest of 84.3% (w/w) glucose and 89.7% (w/w) of xylose.
ro of
However, pretreatment with sulphuric acid resulted in an average of 1.3 times less formation of furfural and phenolic compounds compared to HNO3 [61]. Pretreatment using hydrochloric acids in combination with other techniques has been observed to improve the sugar yield
-p
quite efficiently. In an addition hydrochloric acid requires less temperature than sulphuric acid thus reducing the chance towards the formation of inhibitory by products from the
re
degradation of sugars [64]. Sole efficacy of using 1% (w/w) of hydrochloric acid during the
lP
pretreatment of corn stover for 40 min produced 1.6 times more glucose yield than the native biomass after enzymatic hydrolysis. Also, glucose yield reached a maximum of 71.5% when HCl pretreatment of biomass was immediately followed by ammonia wet oxidation
na
pretreatment at 120 °C temperature for 40 min [65]. In another experiment by Chen et al. the sequential investigation of hydrochloric acid and ionic liquid pretreatment of poplar wood
ur
substantially increased cellulose accessibility and finally leads to a maximum fermentable
Jo
sugars yield of 87.2% after 72 h of enzymatic hydrolysis [66]. Apart from sulphuric, nitric and hydrochloric acids, pretreatment with acetic, citric, oxalic acids have also been observed to effectively increase biomass hydrolysis. Rattanaporn et al. reported that, 10% (w/w) acetic acid significantly improved the saccharification of oil palm shell at a temperature of 100 °C for 30 min. Compared to untreated one, acetic acid treated oil palm shell produced a maximum of 40.4 mg of reducing sugars/g of biomass [67]. In an another experiment mild
20
acetic acid concentration of 3, 7 and 11 g/L was used for the pretreatment of switch grass and at 170 °C for 20 min of reaction period, the residual glucan content has been increased at an average of 33.1% than untreated biomass [68]. Additionally, citric acid was also reported as an effective pretreatment agent specially for algal species. During citric acid catalyzed pretreatment of macro algae Gracilaria verrucosa, 0.1 M citric acid was able to produce 51% (w/w) of total reducing sugars at 150 °C for 60 min [69]. In addition to acetic acid and citric acid, oxalic acid also solubilizes the hemicellulose fraction of biomass on a great extent. The
ro of
severity of the pretreatment was reported at 82 M of acid concentration at 160 °C for 58 min when oxalic acid was incorporated with yellow poplar biomass [70]. Due to a rapid degradation of xylan content, crystallinity of pretreated biomass was increased till a certain
-p
pretreatment severity.
During acid pretreatment, a number of unwanted chemicals are produced in the reaction
re
mixture known as inhibitors for fermentation process. Under protonated condition and at
lP
prolonged reaction period, the produced monomeric sugars liberated three molecules of water for the formation of furan compounds. Furfural is generated as furan from five carbon
na
pentose sugars like xylose and arabinose whereas, hydroxymethyl furfural is produced from six carbon hexose sugars like glucose, galactose and mannose [71]. From furfural, acetic acid is produced and from hydroxymethyl furfural, both of the acetic acid and levulinic acid can
ur
be produced. Apart from furfural and hydroxymethyl furfural, acetic acid is also produced
Jo
from the acidic hydrolysis of xylan backbone of hemicellulose. Detoxification of inhibitors mandates an elimination of theses inhibitors from the reaction mixture prior to the process of fermentation. Short residence time and high production of sugars are the advantages of using acid pretreatment of biomass and dis-advantages include significant removal of hemicellulose and formation of unwanted inhibitors in the reaction mixture.
21
2.2.2. Pretreatment of lignocellulosic biomass using alkali Pretreatment with the incorporation of alkali facilitates substantial lignin removal and subsequently manifests towards more porous structure formation. Besides, alkaline pretreatment improves the accessibility of cellulose by disrupting a certain portion of the biomass. Yang et al. studied the effect of alkali on delignification of bamboo in a combined treatment of hot water and sodium hydroxide at a range of 0.5-2% (w/v) [72]. It was observed that, compared to raw biomass, content of lignin was substantially decreased with each of the
ro of
increase in alkaline concentration and temperature of the reaction. Still, in some occasions the residual lignin in the pretreated biomass impaired the hydrolytic efficiency of biomass. Li et al. observed that, recovery of cellulose and hemicellulose reduced after 70 °C temperature
-p
when sugarcane bagasse was pretreated with NaOH and in the presence of H2O2 as an oxidative agent [73]. Besides, significant lignin removal of ~65% was observed for sugarcane
re
bagasse when the reaction temperature was at fixed at 50 °C followed by an insignificant
lP
lignin removal with further increase in temperature. Further an optimum lignin removal of around 70% was observed at 1% (w/v) of alkaline concentration. Hence, an efficient process of pretreatment is evaluated on the basis of removal of lignin from the biomass which
na
attributes towards an enhancement in the cellulosic conversion. In view of an increased lignin removal a sequential acid and alkali treatment was implied on A. salmiana leaves and a post
ur
acid pretreatment was revealed with significant hemicellulose solubilization. Compared to
Jo
biomass, an increase of ~137% in lignin removal was reported when acid pretreated biomass was subjected to 3.4% (w/v) of NaOH pretreated for 70 min [74]. Effective removal of lignin as a result of alkaline pretreatment makes cellulose and hemicellulose accessible towards enzymatic hydrolysis and attributes towards enhanced sugar production. However, a large proportion of biomass solubilization is observed as major limitation of the process.
22
2.2.3. Pretreatment of lignocellulosic biomass using ionic liquid Pretreatment with ionic liquids has been a major attention in the field of lignocellulosic research as the reaction medium is consisted of organic cation and inorganic anion or cation. Most of the investigations since its exploration, ionic liquids have been observed to showcase few of its tremendous properties such as high thermal stability, less vapor pressure, nonflammability [75]. Due to the low vapor pressure, ionic liquids limit the emittance of volatile components hence considered as green solvents. The potential of various ions presents in
ro of
ionic liquids manifested an effective attempt to dissolute cellulose and solubilize most of the hemicellulose. Ionic liquids dedicated to interact with hydroxyl groups of cellulose are often studied to disrupt the intra and intermolecular hydrogen bonds [76]. Among different ions,
-p
ionic liquid consisted of pyridinium and imidazolium is proven as one of the suitable compounds to efficiently dissolve cellulose. Sashina et al. observed the superiority of 1, 3-
re
disubstituted pyridinium salt as an efficient cellulose dissolution medium over, 1,2-
lP
disubstituted pyridinium. [77]. Saher et al. studied the ability of 1-butyl-3-methyl pyrodinium chloride to effectively dissolve 28% cellulose at 110 ºC [78]. Trinh et al. in an another study of pretreatment revealed that, the potential of 1-butyl-3-methylimidazolium chloride towards
na
significant cellulose digestibility with a fold increase of 29 and 20 for hard wood and soft wood respectively [79]. Miranda et al. addressed the efficiency of different protic ionic
ur
liquids to facilitate lignin removal of pine apple crown and it was studied that bis-2-
Jo
hydroxyethyl ammonium propionate attributed towards 90% of lignin removal in the pretreated biomass [80]. Hence, pretreatment of biomass using ionic liquids has been considered as green technology with respect to biomass dissolution towards subsequent accessibility for enzymatic hydrolysis. Several advantages of ionic liquid pretreatment are effective dissolution and significant delignification of biomass on the contrary, separation of
23
desired product and detoxification of the inhibitors are the major disadvantages of the process. 2.2.4. Pretreatment of lignocellulosic biomass using organic solvents Organosolv pretreatment has been considered as another promising approach of biomass fractionation mainly focused on delignification. On an addition, removal of lignin with satisfactory solubilization of hemicellulose attributes towards more effective accessible area within the biomass for subsequent hydrolysis process. Substantial disintegration of biomass
ro of
depends on type of solvent used. Organosolv treatment employs a number of organic solvents such as ethanol and methanol. During the interaction with the biomass, degradation of lignin manifests towards breakdown of α and β aryl of ether linkages in one type of reactions
-p
followed by the disruption of glycosidic bonds in major part of hemicellulose and cellulose for the conversion into oligosaccharides [81]. Over the recent few years’ deep eutectic
re
solvent has emerged as potential delignification medium due to its high biocompatibility and
lP
bio sustainability [82]. Also, it is highly considered as a perfect alternative for ionic liquids. Further, research focus is more likely inclined towards limiting high viscosity of eutectic solvents. In view of diluting the solvents, New et al. studied, the effect of water in the
na
preparation of eutectic solvent as choline chloride: urea at 1:2 ratios during delignification of oil palm fronds and 30 % (v/v) was found optimum to conduct 16.3% of delignification [83].
ur
In an another experiment, Yu et al. developed a modified hot water pretreatment process
Jo
based on water: choline chloride at 2:1 ratio which leads to a maximum lignin removal of 53.6% from leave sheaths with a minimum consumption of solvents [84]. Chen et al. synthesized an organic solvent mixture of various hydrogen bond donors and acceptors and it was evidenced that, the mixture of guanidine hydrochloride as H-bond acceptor with ethylene glycol and p-tolunesulfonic acid as H-bond donor was the most effective to remove 80% of xylan and lignin from switch grass pretreated at 120 ºC only for 6 min of reaction period [85].
24
So, pretreatment of biomass using eutectic solvents as organosolv has advanced a mean of green technologies for sustainable disintegration of the constituents of biomass. Pretreatment using organic solvent is studied as an effective strategy for both hardwood and softwood with high product yield at an introduction of acid. Requirement of an extra streamline for recovery of chemicals, high process cost are the disadvantages. 2.3. Physico-chemical pretreatment Physico-chemical pretreatment refers to the application of high pressure and temperature to
ro of
the biomass followed by sudden release of the pressure which brings about structural changes through the disruption of intra and intermolecular linkages. Among physico-chemical pretreatments, steam explosion, liquid hot water and wet oxidation are mostly investigated.
-p
2.3.1. Steam explosion pretreatment
Steam explosion has one of the crucial pretreatment technology to promote the breakdown of
re
lignocellulosic matrix at high pressure in the range of 1 to 5 MPa of pressure generated by
lP
saturated steam and high temperature in the range of 150 to 300 ºC within the reactor for a short span of time [86, 87]. As a result of severe pretreatment conditions, substantial disruption of biomass matrix attributes towards partial removal of lignin and the process like
na
auto-hydrolysis mediating through the solubilization of hemicellulosic sugars. Partial removal of lignin manifested towards the formation of pseudo lignin over biomass surface. So, in view
ur
of reducing the lignin content, addition of chemicals facilitates the process of substantial
Jo
delignification during steam explosion pretreatment of biomass. Veradi et al. observed that the efficiency of steam explosion pretreatment was significantly improved when sugar cane biomass was impregnated with 0.2% and 1% (v/v) of hydrogen peroxide solution and an average increase of 12% and 34% in glucose yield from H2O2 impregnated biomass was observed compared to untreated one [88]. In an another steam explosion pretreatment, 0.5% (w/w) H2SO4 was impregnated on elephant grass and at optimized reaction temperature of
25
161 ºC, the treated biomass was obtained with 52.1% (w/w) of cellulose after 11.5 min of time which finally attributes towards 55% of scarification yield after enzymatic hydrolysis [89]. Wang et al. also reported substantial presence of monosaccharides in the pretreated filtrate when spruce was impregnated with both of sulphuric acid and sulphur-di-oxide. Pretreated filtrate obtained from biomass using both of the impregnating reagent resulted into significant inhibition towards cellulolytic enzymes during enzymatic hydrolysis [90]. Steam explosion treatment is observed as effective against hardwood and agricultural residues.
ro of
However, high hemicellulose removal and ineffectiveness against softwood biomass impedes the technology from practical application. 2.3.2. Liquid hot water pretreatment
-p
Liquid hot water also designated as hydrothermal pretreatment has been well known as green technology to disintegrate the intricate structural matrix of biomass. Furthermore, the process
re
involves compressed water as the solvent with no extraneous addition of any chemicals. So,
lP
the evolved energy and cost of the process is comparatively less and the feasibility towards the formation of inhibitors is less. Moreover, selective hydrolysis of hemicellulose mediating through the formation of hydronium ions partially interfere with the alterations of
na
lignocellulose and the formation of pseudo lignin with unwanted inhibitors vastly disturb the process of enzymatic hydrolysis. On an addition, even at the higher temperature of 200 ºC,
ur
reallocation of lignin was vigorously reported on the surface of biomass fibers manifested
Jo
towards more residual lignin. Consequently, different attempts are made to determine the optimized hydrothermal condition for different biomass. Batista et al. hydrothermally pretreated sugarcane straw at different reaction times and temperatures and the severity of the reaction for temperature is observed as dominant over reaction time. At optimized reaction condition of 195 ºC for 10 min, 58.8% (w/w) of cellulose was obtained in the pretreated biomass compared to the existence of 33.1% (w/w) of cellulose in the raw sugarcane straw
26
and further 85.5% (w/w) of hemicellulosic sugars are also reported in the pretreated filtrate [91]. Haldar et al. reported the potential of hot water pretreatment to prepare the feedstock obtained from waste banana stem for enzymatic hydrolysis and an incorporation of 20 FPU cellulase enzyme resulted into the production of 12.8 g/L of total reducing sugars [92]. Lyu et al. conducted two stage liquid hot water pretreatment using cassava straw as lignocellulosic biomass and at the optimized reaction conditions (180 °C for 60 min in first stage and 200 °C for 20 min) ~83.1% (w/w) of pentose sugars was obtained with a substantial yield of ~85%
ro of
(w/w) of hexose sugars from enzymatic hydrolysis [93]. Hence, liquid hot water treatment facilitates towards maximum hemicellulose removal with substantial cellulose recovery in the residual biomass and also attributes towards less formation of inhibitory compounds as side
-p
product. The advantages of liquid hot water treatment are no requirement of adding any external chemicals and maximum portion of lignocellulosic contents are retained in the
re
pretreated biomass whereas, the disadvantage is an insignificant removal of lignin from both
lP
of hardwood and softwood. 2.3.3. Wet oxidation pretreatment
Wet oxidation has been another physicochemical pretreatment method in which biomass is
na
treated in the presence of oxygen or air as an oxidant and at the temperature in the range of 100 to 200 °C. The applied pressure is varied up to a maximum of 20 MPa for residence time
ur
in the range of 0.5 to 2 h [94]. Moreover, instead of oxygen, incorporation of other chemical
Jo
as oxidant leads the process cost effective. Therefore, uses of oxygen or air as an oxidant is mostly preferred during any wet oxidation treatment. During the treatment, breaking down of the solid crystalline matrix of cellulose facilitates hemicellulose conversion from solid polymeric phase to liquid oligomeric phase. Balat et al. showed that, at temperature more than 170 °C, lignocellulosic biomass undergoes autohydrolysis and attributes towards formation of organic acids which leads to bring down the pH of the medium in the range of 3
27
to 4.5 [95]. However, more acidic pH of the reaction medium leads to produce hydroxyl methyl furfural and basic medium leads to produce aldehyde compounds [96]. Table 5 highlights on the recent advancements of different physicochemical pretreatment methods for various lignocellulosic residues. The table highlights on the mostly explored lignocellulosic biomass such as rice straw, wheat straw, sugarcane bagasse, corn stover and sorghum bagasse and it can be briefed that, severity of the effect of pretreatment depends on the nature and type of the biomass. Biomass treated with alkali or ionic liquid shows substantial
ro of
improvement in sugar yield. Efficient removal of lignin and low formation of inhibitors are the merits of wet oxidation of biomass while high cost involved with oxygen and catalyst limit the process from its ground applicability.
-p
2.4. Biological pretreatment
Most of the pretreatment processes discussed earlier are associated with high energy
re
consumption or economically unsuitable. However, less energy input and minimum
lP
requirement of chemicals established biological pretreatment process as a sustainable alternative for lignocellulosic disruption. In view of lignocellulosic disintegration, lignin has been an essential constituent of biomass as it heavily cross linked within the intricate network
na
of cellulose and hemicellulose. A number of white rot fungi have been discovered with potential ability to breakdown the lignin of the biomass through the production of ligninolytic
ur
enzymes. Deswal et al. studied the maximum production of ligninolytic enzymes at fifteen
Jo
days of solid state fermentation when white rot fungi like P. florida, C. caperata and G. sp. were inoculated into the reaction mixture of sugarcane bagasse. Besides, a highest delignification of 7.9% was obtained for P. florida after 15 days of solid state fermentation. In an another study of biological pretreatment with different biomass, a white rot fungus I. lacteus was observed to bring about highest lignin removal of 42.3% and 45.8% for wheat straw and corn stover respectively [97]. Martinez Patino et al. studied biological treatment of
28
olive tree biomass and compared to raw untreated biomass, a significant increase of 312.5% was observed in glucose concentration after enzymatic hydrolysis of the biomass obtained after a sequential 45 days of continuous pretreatment in the presence of I. lacteus followed by acid hydrolysis with 2% (w/v) H2SO4 [98]. Under the condition, enzymatic hydrolysis reached with a yield of around 50% along with glucose recover of 39.6%. Among the advantages, an effective degradation of lignin and partial removal of hemicellulose at mild reaction conditions manifests less energy consumption whereas, slow reaction rate and
ro of
unsatisfactory yield compared to the other available process stands as barrier for biological pretreatment process
Pretreatment is regarded as a crucial unit operation in order to break the outermost shell of
-p
lignin for making cellulose and hemicellulose accessible to enzymatic hydrolysis process. In view of that, pretreatment of biomass using hot water may be a promising approach for the
re
biomass with low lignin content as the process doesn’t demand of any extraneous addition of
lP
chemicals to disrupt the structural intricacy of lignocellulosic residues. However, alkaline treatment facilitates substantial delignification of the biomass with high content of lignin. 2.5. An insight on latest and advanced emerging pretreatment technologies
na
The conventional methods of pretreatment processes described in preceding section is focused primarily on the breakage of the biomass recalcitrance. However, these techniques
ur
suffers severely from an effective and practical solution with regard to an industrial
Jo
adaptation [99]. Interestingly in the recent years, with the advent of applied research, a number of promising approaches are emerged as green technologies for the pretreatment of lignocellulosic biomass. Among the others, few of the emerging technologies involve an irradiation of microwave, electron beam and gamma ray along with the application of pulsed electric field and high hydrostatic pressure as advanced form of pretreatment methods for biomass disintegration. During pretreatment, an electromagnetic energy of microwave
29
converts well into heat and dissipated uniformly throughout at the molecular level within the biomass particles manifests consistent disruption as an effect of microwave irradiation. Recently, microwave assisted alkali treatment with 0.5% (w/v) of NaOH was reported as the best suited method for pretreatment of brewers spent grain when 1 g of biomass was treated at 400 W only for 60s leads to the production of 20% (w/w) of reducing sugars manifests ~3 fold increase compared to an untreated biomass [100]. During electron beam irradiation, ionizing radiation is generated from linear accelerator for the formation of free radicals, once
ro of
the biomass is exposed into the reaction system. In view of that, 500 kGy of electron beam was used to depolymerize M. sinensis for an efficient production of fermentable sugars [101]. Application of pulsed electric field facilitates delignification and porous formation within the
-p
surface of biomass attributes towards an easy penetration of hydrolytic enzymes through the pores. Kumar et al. studied the effectiveness of pulsed-electric field when wood chip and
re
switch grass was pretreated under the influence of 2000 pulses and at field strength of 10
lP
Kv/cm resulted into an enhanced conversion of biomass into value-added products [102]. The concept of introducing high hydrostatic pressure in the range of 100 to 600 MPa on lignocellulosic biomass is subjected with an aspiration of uniform distribution of pressure and
na
as a result, 5-10 fold increase in initial rate of xylan hydrolysis was observed when E. globulus pulp was exposed towards 300-400 MPa hydrostatic pressure for 45 min [103].
ur
Raud et al. studied an effective and novel decompression pretreatment method for
Jo
lignocellulosic biomass under nitrogen explosion in the range of 1-3 MPa at 150 °C with no addition of any extraneous chemicals [104]. Further, Ravindran et al. investigated another novel FeCl3 assisted plasma pretreatment for spent coffee waste and as a result, 0.496 g of reducing sugars per g of treated biomass attributed into the production of 18.6 g/L of ethanol when the hydrolysate was fermented in the presence of S. cerevisiae [105]. The emerging pretreatment methods are considered as sustainable approaches for subsequent hydrolysis of
30
lignocellulosic biomass due to the promising ability of the technologies to offer maximum operational output at an expense of minimum efforts. The advancement in lignocellulosic biomass hydrolysis is discussed in details in subsequent sections.
3. Strategies towards improved enzymatic hydrolysis of biomass On an economical perspective, enzymatic hydrolysis has been a deciding factor towards an efficient conversion of biomass to biofuel. Following a number of various pretreatments, the solid biomass is subjected to rigorous enzymatic hydrolysis in the presence of cellulase,
ro of
hemicellulase and β glucosidase enzymes [106]. Microbial enzymes extracted from different microorganisms have been experimented on a variety of biomass to obtain the maximum yield in an optimum reaction condition. Further, enzymatic hydrolysis is mostly preferred as
-p
the process is not associated with the toxic productions of inhibitors thereby save the accessory cost including detoxifications of inhibitors. Moreover, high substrate concentration
re
facilitates towards better yield with an expense of more enzyme consumption [107].
lP
Nevertheless, incorporation of high dose of enzyme excels the cost of the process and hamper the sustainable production of biofuels. As a result, selection of proper enzyme cocktail with
na
an identification of optimized reaction condition has been a challenging aspect without compromising the product yield. Table 6 explains the effectiveness of using enzyme cocktail
ur
towards an improved enzymatic hydrolysis. From the table, it can be said that when cellulase enzyme was provided with other accessory enzymes then the enzyme cocktail shows
Jo
substantial improvement in the hydrolysis of biomass. Enzymatic hydrolysis is mostly conducted with cellulase enzyme extracted from Trichoderma reesei [108-110]. Deficiency of β-glucosidase activity in cellulase enzyme increased the accumulation of cellobiose attributes towards product inhibition. Hence, incorporation of β-glucosidase enzyme with cellulase enzyme improved the product yield obtained from enzymatic hydrolysis of biomass [111, 112]. In order to compensate with high cost of enzymes, different strategies were made 31
to improve the saccharification yield. In view of that, addition of additives such as Triton X100 and Tween 20 have been exclusively used and showed an increase in the hydrolysis rate by improving enzyme stability and activity [113]. Martin et al. studied the effect of polyethylene glycol on the hydrolysis of pretreated biomass and subsequent study revealed that, an average of 8.8% increase in glucose yield was observed when substrate was corn stover and sugarcane straw with 32% increase for avicel [114]. Likewise, in an another study alkalophilic fungus MVI2011 was developed to produce substantial ligninase enyme for a
ro of
rapid disintegration of rice straw and subsequent study on submerged cultivation for seven consecutive days’ attributes towards 74.2% increase in saccharification yield. Further, from compositional aspect, as glucan and xylan are the major polysaccharides present in the
-p
biomass so integrated activity of cellulase and xylanse has been beneficial towards an effective hydrolytic conversion. Chen et al. developed a bi-functional enzyme
re
cellulase/xylanase (CtCel5E) derived from C. thermocellum and fused with β-glucosidase
lP
(CcBgIA) from C. cellulovorans which resulted into an improved production of glucose from carboxymethyl cellulose and rice straw [111]. During enzymatic hydrolysis, a prompt release of product is naturally obtained at very starting phase of hydrolysis which was followed by a
na
sluggish rate in production. The decrease in the production manifests towards two different possibilities. One, consumption of substrates with the reaction leads to decrease the
ur
production and other is the accumulation of products, interfere with the action of enzymes
Jo
and process of hydrolysis. Haldar et al. studied the inhibitory effect of different monosaccharides (glucose, xylose, galactose, mannose and arabinose) disaccharide (cellobiose) and inhibitors (furfural, acetic acid and HMF) through the external additions into the reaction mixtures during enzymatic hydrolysis of waste banana stem and it was observed that, apart from arabinose, each of the external additions leads to inhibit the enzymatic system of cellulase commercialized from T. reesei [92]. Hence, continuous monitoring of the
32
concentration of sugars in the hydrolysate provides an insight to tackle the product inhibition. In an enzymatic inhibitory concept, other than product, substrate inhibition is an important factor to analyze the decrease in the production. More substrate leads to produce more sugars in any conventional enzymatic system. However, during enzymatic hydrolysis, the increase in production with the increase in substrate concentration forms an asymptote at certain concentration of the substrate, which is immediately followed by an insignificant change in the production even with the increase in substrate dosage-known as substrate inhibition.
ro of
Unavailability of substrate binding site for the active site of the enzyme manifests towards an insignificant change in production.
Like cellulose, enzymatic hydrolysis of hemicellulose is another important concern as the
-p
lignocellulosic biomass is primarily consisted of 25-30% (w/w) of hemicellulose. Xylan is the major backbone of hemicellulosic fraction of biomass and xylanase has been the prime
re
enzyme for hydrolyzing xylan for the production of xylose at favorable reaction conditions.
lP
Further, a synergistic working mechanism of endo-xylanase, exo-xylanase and β-xylosidase breaks the hemicellulosic network of any lignocellulosic biomass. Endo-xylanase acts internally on the main chain of xylan polymer to produce xylo-oligomers whereas exo-
na
xylanase cleaves externally on the reducing ends and finally β-xylosidase produces xylose from xylo-oligomers. Like xylanase arbinosidase produces arabinose after hydrolyzing
ur
arabinan polymer of lignocellulosic biomass.
Jo
3.1. Factors influence the process of enzymatic hydrolysis A number of different substrate and enzyme related factors influence the process of enzymatic hydrolysis. Among the substrate related factors cellulose crystallinity is most important in assessing the rate of enzyme enzymatic hydrolysis. It was observed that, process of pretreatment is applied to reduce cellulose crystallinity by decreasing the particle size of the biomass. However, some commercial cellulase mixture is observed to effectively
33
hydrolyze crystalline cellulose [115]. Apart from cellulose crystallinity, the availability of substrate binding site for the active site present in the enzyme influences the progress of enzyme hydrolysis. The purpose of pretreatment is aimed to increase the surface area available for enzymatic hydrolysis. Further, lignin stands as major barrier for hydrolysable substrate fraction towards enzymatic hydrolysis. Apart from, the non-productive binding of lignin on enzyme typically retards the ability of active site to bind with accessible region of substrate molecule [116]. A number of different strategies including addition of protein and
ro of
additives are investigated to overcome the problem associated with non-productive adsorption of lignin onto cellulase [117]. Finally, an increase in porosity and reduction in particle size significantly improves the process of enzymatic hydrolysis.
-p
3.2. Cellulase enzyme: Biochemistry and mechanistic interactions
Primarily, cellulase enzyme involved in the hydrolysis of biomass is mostly isolated from
re
Trichoderma sp. [108, 110]. In view of that, cellulolytic mixture particularly obtained from
lP
Trichoderma sp. is believed to act synergistically for an effective disintegration of polymeric carbohydrates. Exoglucanases, CBHI (Cellobiohydrolase-I) and CBHII (CellobiohydrolaseII) releases cellobiose molecules from the end of cellulosic chains and endoglucanases cut the
na
chain at inside of any places within the cellulose. While β-glucosidase finally relieves the glucose molecules out the produced cellobiose [118, 119]. Hence, an instantaneous
ur
breakdown of cellobiose into glucose molecules avoids the feasibility of product inhibition by
Jo
cellobiose molecules. Fig. 4 depicts the synergistic action of cellulase enzyme to produce glucose molecules from cellulose. Now, as per as progress of the hydrolytic reaction is concerned, the combined activity of cellulose binding domain (CBD) and catalytic domain (CD) have been one of the two most critical factors. CBD facilitates the binding of cellulase enzyme on the surface of cellulose containing biomass while CD activates the catalysis involved in the hydrolytic production of glucose molecules. Any conformational changes
34
within these domains lead to impair the progress of hydrolysis. With regard to an improper interactions between –OH group of sugars with –CONH2 group of CBHI leads towards conformational change within the active site of the exo-glucanse enzyme [120]. Cel7A has been the most termed exoglucanse as the CBHI accounts around 60% of total protein secreted by Trichoderma reesei. A number of different aromatic amino acids such as tyrosine, phenylalanine and non-polar amino acids like leucine, isoleucine was found distributed over the surface of the enzyme. Glu212 and Glu217 was observed as the two most active amino
ro of
acids which takes part in the formation of an intermediate enzyme-cellobiose complex mediated through nucleophilic reaction. Finally, subsequent protonation of cellobiose molecules manifested towards the production of glucose from the intermediate complex
-p
[121].
3.3. Efforts to overcome the criticalities involved with cellulase
re
A number of different factors involved with cellulase enzyme limits full fledge
lP
commercialization of enzymatic hydrolysis of lignocellulosic biomass. Among the others, high cost of enzyme along with the performance on substrate molecules is often considered as the most dictating factors of the overall process of enzymatic hydrolysis. Reduction in the
na
cost of the enzyme is in line with the requisite support to provide an optimum growth condition for the microbes towards the production of cellulase enzyme. In view of that, an
ur
introduction of submerged fermentation or solid-state fermentation using substrate specific
Jo
microbes are widely reported [122]. To reduce an overall cost of the process, enzyme immobilization is recognized as the most effective strategy to reuse the enzyme for repetitive times with almost similar activity [123]. Further, the problem of feedback inhibition due to an instant accumulation of cellobiose during the hydrolysis of cellulose. In order to overcome the accumulation of cellobiose, supplementation of β-glucosidase in cellulase is studied to limit the restriction on enzyme-cellulose complex caused by cellobiose molecules.
35
Moreover, apart from acting as a barrier for cellulolytic enzymes, lignin is observed to involve in non-productive binding with the amino-acids present in the active site of cellulase enzyme thus manifesting a reduction in enzymatic production of sugars. A number of different strategies including an addition of additives such as surfactants and proteins are observed to significantly improve the rate of enzymatic production through the binding with lignin molecules [117, 124].
4. Strategies involved in the fermentation of sugars into biofuel
ro of
Primarily, the hydrolysate obtained after chemical and enzymatic hydrolysis has been used as feedstock material for fermentation process. In view of that, a number of fermentation strategies studied for different lignocellulosic biomass using a horizon of microorganisms.
-p
Table 7 highlights on few of the top most companies around the globe working in the area of biofuel using low cost biomass. Among them, Abengoa bioenergy has recently made
re
significant breakthrough using a novel enzymatic process towards the production of 25
lP
million gallon of ethanol per year followed by an annual production of 42 million liters of bioethanol by Raizen-Iogen at the unit located in Brazil. Out of the several strategies,
na
separate hydrolysis fermentation (SHF) and simultaneous saccharification fermentation (SSF) are mostly used during the fermentation of hydrolysate obtained from lignocellulosic
ur
biomass. With regard to SHF, SSF is mostly preferred due to the ability of the system to overcome the problem of product inhibition [125]. Still, the difference in the optimum
Jo
reaction temperatures for hydrolysis and fermentation demands technological improvements in order to facilitate the progress during the process of fermentation. With regard to that, an incorporation of thermo-tolerant microbes seems to be adaptable even at high temperature required for enzymatic hydrolysis. Now as per as fermentation product is concerned, biobutanol has received much attention over bioethanol as the former contains high energy content, lower heat of vaporization, less corrosive, substantial intermixing property [126]. 36
Most of the recent research investigations on biobutanol production acknowledge ABE pathway
using
species
such
as
Clostridium
acetobutylicum,
C.
beijerinkii,
C.
saccharobutylicim [127, 128]. 4.1. Consolidated bioprocess (CBP) involved in fermentation The term consolidated arises as the entire production process of biomass to biofuel is accomplished into a single reactor. During this process, microbes produce enzyme, which leads to disintegrate the biomass itself for the production of biofuel within the reactor [129,
ro of
130]. Hence, on an economical perspective, the process of consolidated bioprocess has emerged as one of the promising approaches compared to the process in which enzyme production, saccharification and fermentation are carried out separately. Jiang et al. carried
-p
out CBP using a new thermos anaerobacterium sp M5 for the xylan as substrate and at 55 °C temperature, 1.1 g/L of butanol was produced which was enhanced to 8.2 g/L when a mix
re
culture of thermoanaerobacterium sp M5 and C. acetobutylicum was employed in CBP
lP
[131]. In an another approach, consolidated bioprocess was employed to produce bioethanol with a yield of around 67% from the slurry obtained from dilute acid treated wheat straw of 17 g/L of substrate concentration [132]. The effectiveness of consolidated bioprocessing
na
shows an isopropanol production of ~8.4 g/L when consortium of EMSD5 was explored for the conversion of arabinoxylan of corncob [133].
ur
4.2. Simultaneous saccharification and co fermentation
Jo
Simultaneous saccharification and fermentation is carried out with a set of microorganisms which possess the significant capability to ferment pentose and hexose sugars together at same reaction conditions. Liu et al. carried out simultaneous saccharification and co fermentation of S.cerevisiae and C. tropicalis at different ratios, temperature and pH. Under optimum reaction conditions of pH 5, temperature of 32 °C in 144 h of reaction period a maximum of 11 g/100 mL of bioethanol was obtained [134]. So, microorganisms are well
37
capable of an efficient utilization of mixture of pentose and hexose sugars present in biomass hydrolysate for the production of biofuels. However, few microbes are reported as incapable of utilizing the pentose sugars present in the feedstock material for fermentation [135]. Although E. coli exhibits an inherent property for a parallel utilization of both of the pentose and hexose sugars as substrate though, along with other engineered microorganisms E.coli was also observed to show the phenomenon of carbon catabolite repression (CCR). In view of that, CCR is known as metabolic regulation of any microorganism to exhibit their
ro of
preferences of using a particular sugar over others present in the hydrolysate sample, which ultimately attributes towards a reduction in the production of ethanol at the end. In order to mitigate such bottleneck of CCR, continuous culture facilitates towards a significant
-p
consumption of sugars and reduction in fermentation time. Fernandez-Sandoval et al. conducted single stage continuous cultures using micro-aerated conditions and as a result,
re
steady state ethanol production of 18 g/L was observed when glucose of 7.5 g/L and xylose
lP
of 42.5 g/L was consumed together as a mixture in mineral medium [136]. Although coconsumption of multiple sugars as carbon sources may be improved in microorganism after modulating the genes including mgsA, pgi in E. coli involved in phosphotransferase systems
na
(PTS) but due to such anticipation, ethanol production was negatively affected. On the contrary, modulation on the genes such as; galP, glk of non PTS pathways was studied to
ur
exhibit an improved production in cellulosic ethanol [137]. Further, inactivation of gid and
Jo
ccp genes directs carbon catabolite repression in Clostridium sp. MF28 with a simultaneous ability to co-ferment glucose, xylose and arabinose together for the production of around 12 g/L butanol [138]. 4.3. Simultaneous pretreatment and saccharification During biological pretreatment, few microorganisms have been observed to deconstruct the structure of lignocellulosic biomass and subsequent saccharification into monomeric sugars.
38
In this regard, an attempt to find the ligninolytic fungi with cellulolytic enzymes may be beneficial for improved saccharification of biomass. In this particular phenomenon of coexistence, different reaction conditions of delignification and enzymatic saccharification negatively impact the overall saccharification performance of the process and the separation of fungi has been another concern after the pretreatment process [139]. Karimi et al. studied simultaneous delignification and saccharification of rice straw by immobilized Trichoderma viride and under optimal reaction condition, lignin removal efficiency of around 74% and 8.5
ro of
g/L sugars were obtained after 10 days of pretreatment [140]. Moreover, a number of different microbes such as S. cerevisiae, Z. mobilis, P.stipitis are commonly used during the fermentation of biomass. Among different microbes, few microbes like S. cerevisiae is well
-p
known to ferment hexose sugars while Pichia sp is specialized for pentose sugars. An efficient conversion of biomass to biofuel demands involvement of microbes able to ferment
re
both of the pentose and hexose sugars. Hence, genetic modification in different
lP
microorganisms are immensely focused to improve the fermentation ability for both pentose and hexose sugars together in a single reaction system. Table 8 briefed on various fermentations strategies including merits and demerits of the individual methods.
na
4.4. Exploration of metabolic engineering towards improved biofuel production Commercial installation of fermentation technology has been impeded due to some existing
ur
bottleneck of limited yield and inappropriate substrate consumption by the microbes. In order
Jo
to rectify the concerns, understanding the metabolism or genetic modifications of the organism has been an important aspect. As per as large scale fermentation is concerned, S. cerevisiae and Z. mobilies are among mostly employed microorganisms for the lignocellulosic ethanol production [141-144].
39
4.4.1. Engineered saccharomyces cerevisiae for bioethanol production Apart from the ability of producing high yield of ethanol from a number of lignocellulosic biomass, S. Cerevisiae has also been observed to exhibit the properties of high tolerance against osmotic pressure and viral infections [145, 146]. The fungal strain has been studied with few limitations when substrate is xylose. On the other side, around 35 to 40% of the feedstock is consisted of xylose. So, in order to increase rapid xylose fermenting ability of the strain, glucose repression on xylose fermentation need to overcome. With regard to that, most
ro of
of the natural yeast strain has been observed to ferment xylose under anaerobic condition thus not amenable to industrial context. Hence, development of an industrial strain has always been a top research endeavor to overcome the existing issues. Though, most of the industrial
-p
strain has been observed as polyploidy thus attributes to perform sporulation and HO gene deletion to convert them into haploid or diploid which often losses the advantageous trait of
re
polyploids [147]. To get rid of such intricacies, recent development of Cas9 based genome
lP
editing technology has been developed to insert and delete target gene within the polyploid yeast strain [148]. Lee et al. introduced Cas9 based genome editing on S. cerevisiae strain to construct an auxotrophic engineered strain, JX123 after incorporating xylose metabolic
na
pathway. Additionally, the accumulation of xylitol was significantly reduced through NADH oxidase from L. lactis [141]. Moreover, simulation based study revealed that, glucose act as a
ur
potent inhibitor for S. cerevisiae during xylose uptake when substrate medium is consisted of
Jo
both of the sugars [149]. Despite of numerous attempt to advance genetic engineering, glucose transporters exhibited limited affinity to transport xylose through facilitated diffusion within the cell [150]. Hence, both are consumed simultaneously at glucose limited conditions. Lane et al. studied genome sequencing of a mutant stain and reported that, mutation in glucose phosphorylating enzymes such as hexokinase1, hexokinase2 and glucokinase1 manifested simultaneous uptake of glucose and xylose [151]. Likewise, Farwick et al.
40
reported that, mutations at asparagine and threonine amino acids located at transmembrane Gal2 and Hxt7 transporter substantially facilitate the transport of xylose without any inhibition caused by glucose [152]. 4.4.2. Engineered Zymomonas mobilis for bioethanol production Among different fungi, Zymomonas mobilis has been one of the well-recognized ethanologenic microorganism to efficiently convert glucose into bioethanol [153, 154]. Also, both the organisms S. cerevisiae and Z. mobilis uses the common platform of bioethanol
ro of
production. However, Z. mobilis uses Entner-Dioudoroff pathway instead of using EmbdenMeyerhof-Parnas pathway (used by S. cerevisiae) for ethanol production thereby consumed less ATP compared to S. cerevisiae. Further, on a specific amount of hexose sugar, Z. mobilis
-p
was studied to produce less biomass compared to other yeast cells. Thereby, based on the numerous advantages, Z. mobilis was considered as model organism for higher ethahol yield
re
[155]. Like S. cerevisiae, this organism is also observed with some limitation during the
lP
fermentation of pentose sugars specially xylose and arabinose. So, a number of different approaches were studied to overcome the fermentation ability of pentose sugars by Z. mobilis. Five different genes encoding araA, araB, araD, talB, tktA was isolated from E.coli
na
bacterium and introduced into Z. mobilis and more than 90% of theoretical ethanol yield was reported utilizing only arabinose as sole carbon source [156].
ur
4.4.3. Genetic engineering in Clostridium acetobutylicum for biobutanol production
Jo
Clostridium acetobutylicum is well established to produce butanol using glucose, xylose and arabinose obtained from lignocellulosic biomass. Primarily, the pentose sugar, xylose with an accompany of glucose constitute the major portions of the hydrolysate obtained from biomass. During the persistence of glucose repression, utilization of xylose mainly depends on xylose isomerase, xylulokinase and enzymes of pentose phosphate pathway [157]. To improve xylose uptake efficiency, gene knock out system was adopted to inactivate glcG
41
gene of D-glucose phosphoenolpyryvate dependent phosphotransferase system. Further, around 24% increase in butanol yield was observed when other relevant genes were coexpressed in the GlcG mutant strain. But, less improvement in xylose uptake still demands further investigations with the engineering of specific xylose transporters. In an another study, around 12 g/L of butanol was produced when the gene encoded the CcpA protein was discarded from C. acetobutylicum ATCC 824 manifested substantial improvement in simultaneous glucose and xylose uptake in the absence of catabolite repression [158].
ro of
5. Conversion of biomass into the derivatives of cellulose and lignin
Growing concern over fossil fuel depletion reflects much attention on sustainable conversion of lignocellulosic biomass into value-added products. Based on the fact, biomass conversion
-p
has been immensely acknowledged at everywhere in academics and industries for a valuable transformation of biomass into fuel and platform chemicals [24, 159]. Over the years,
re
lignocellulosic biomass has been the cheap, abundant and most viable source to gather
lP
cellulose, hemicellulose and lignin in its polymeric form. Among these, cellulose and hemicellulose represents around 70-75% of total biomass and most of the rest is consisted of
na
polymeric phenols such as lignin. Now conversion of cellulose into glucose, gluconic acids, HMF, lactic acid and levulinic acid has already investigated and received massive
ur
acknowledgement [160]. Nevertheless, promising approaches involving greener routes towards the production of sustainable fuel and chemicals from cellulose and lignin are briefed
Jo
over here.
5.1. Derivatives of cellulose Among the derivatives obtained from cellulose, different acids, alcohols and HMF are well known.
42
5.1.1. Formic acid Formic acid is well known as the simplest form of carboxylic acid primarily used as storage form of hydrogen as it can be efficiently converted into carbon-di-oxide and hydrogen through rapid decomposition of formic acid by catalytic hydrothermal reaction [161]. It was apparently seen that, yield of formic acid is much higher from glucose than cellulose [162]. Under protonated condition, cellulose is hydrolyzed into glucose and in presence of an oxidant, glucose is oxidized into formic acid at higher temperature and pressure. Hence,
ro of
oxidative hydrothermal reaction facilitates the formation of formic acid from cellulose or glucose. In the presence of vanadyl ion (VO2+), oxidative hydrothermal reaction of cellulose leads to produce around 65% (w/w) of formic acid from glucose at temperature of 160 °C
-p
[163]. In an another experiment, cotton was alkylated to disrupt glycosidic linkages and a post hydrothermal reaction for 6 h leads to produce a maximum of 21.9% (w/w) formic acid
re
from cellulosic fraction [164].
lP
5.1.2. Acetic acid
Acetic acid is the second simplest form of carboxylic acid and suitable to use as vinegar for household purposes. Similar as formic acid, under strong protonated condition, glucose
na
molecules, the hydrolyzed sugar of cellulose is oxidized into acetic acid. Most of the hydrolysate obtained from the pretreatment of lignocellulosic biomass contains a certain
ur
amount of acetic acid. During acidic hydrolysis, cellulose hydrothermally degraded into
Jo
levulinic acid which further degraded into acetic acid under no oxygen condition [165]. In a similar work by Jin et al., acetic acid was produced using two-step process in which under alkali condition, first lactic acid was produced from biomass by hydrothermolysis reaction and followed by the production of acetic acid in the presence of H2O2 [166]. Still, usage of H2O2 was cost effective with insignificant improvement in the process. Thereafter, Huo et al. replaced H2O2 with CuO and observed an average of 23.7% yield in acetic acid from
43
carbohydrates such as glucose, cellulose and starch [167]. During the interaction with lactic acids, Cu2+ forms coordinate bond with two O atoms of lactic acids thereby decreases electronegativity of hydroxyl group. Further, nucleophilic attack by OH- of alkali on H atom of hydroxyl group of lactic acid subsequently leads to form acetic acid through an intermediate production of acetaldehyde and formic acid (Fig. 5). 5.1.3. Levulinic acid An efficient utilization of lignocellulosic biomass as a renewable feedstock for the production
ro of
of value-added chemicals not only save the cost but also mitigate the problems of greenhouse gasses. Among a number of different chemicals, levulinic acid has received much attention for the preparation of pharma products, polymers, plastic, resins and last but not the least can
-p
be used as fuel with gasoline without the any modification in engine architecture [168]. A number of research investigation has reported that, levulinic acid is produced directly from
re
acidic hydrolysis of lignocellulosic biomass. Among the acids, sulphuric acid is mostly used
lP
at a range of 0.1 to 2 mol/L to catalyze hexose sugars like glucose and fructose. During acid hydrolysis, three molecules of water is removed from hexose sugars as a result of dehydration at a temperature of 140-220 °C followed by a rapid conversion into levulinic acid [71, 169].
na
Jeong et al. showed that, modified zeolite in the presence of 0.25 M NaOH produced 4.7% of levulinic acid from bamboo at 190 °C for 30 min [170]. In an another study, 47.5% yield of
Jo
[171].
ur
levulinic acid was observed when bamboo was treated with acidic ionic liquid at 100 °C
5.1.4. Sugar alcohol Sugar alcohols are of immense importance as it is being widely used in food industries and produced from cellulose and glucose of lignocellulosic biomass through hydrogenation reactions. In view of that, mostly produced sugar alcohols are mannitol and sorbitol catalytically obtained from glucose in the presence of hydrogen [172]. Further metal catalyst
44
such as Pt, Pd and Ru in combinations with sulphuric acids were observed towards a conversion of 60% into C4 and C6 sugar alcohols in an hour when the biomass was spruce [173]. Additionally, direct conversion of softwood (Conifers and gymnosperm trees produces softwood biomass and seeds of this plants are observed as uncovered) cellulose into 5C and 6C sugar alcohols using supported platinum catalyst in water was also studied in a hydrogenolysis reaction [174]. Yamaguchi et al. developed Ru and Pt based metal carbon catalyst which was observed to produce 55.1% (w/w) sugar alcohols directly from ball milled
ro of
hard wood biomass [175]. 5.1.5. Hydroxymethyl furfural (HMF)
Hydroxymethyl furfural (HMF) has been one of the top most valuable intermediate which is
-p
primarily obtained from a number of different lignocellulosic feedstocks. In an addition, HMF is also produced from synthetic monosaccharides, disaccharides and polysaccharides.
re
An application of mostly used catalyst such as minerals and organic acids facilitate towards
lP
an improved production of HMF. Besides, HMF is recognized as “sleeping giant” by some authors due to its innumerable importance to act as valuable intermediate for the formation of a number of platform chemicals. Nguyen et al. reported that, a maximum yield of around 79
na
mol% for HMF was obtained when biomass was first treated with 3% (w/w) NaOH and subsequently catalyzed in a reaction system comprising of 1-butyl-3-methyl imidazolium
ur
chloride ([BMIM]Cl) as ionic liquid and CrCl3.6H2O used as the catalyst [176]. On a
Jo
mechanistic aspect, HMF is mainly produced from acidic dehydration of hexose sugars primarily glucose. In the presence of strong acidic conditions, three molecules of water move out of the glucose molecule and subsequently transformed into HMF.
45
5.2. Derivatives of lignin The cleavage of the predominant C-O type linkages within lignin molecules leads to thermochemical degradation into number of chemicals subcategorized into phenols, aldehydes and acids. 5.2.1. Phenol Hydrogenolysis, acedolysis, depolymerization are among promising methods to convert lignin into value-added products. The effect of hydrogenolysis has been quite rare event for
ro of
lignin conversion and on the other hand a severe problem with lignin depolymerization has been immense as re-condensation of the lignin fragments leads to form non cleavable bonds in insoluble lignin. In order to overcome re condensation of lignin fragments, lignin was
-p
reacted with formaldehyde to form 1,3-dioxane structure thus the condensation reaction is prevented by forming a soluble lignin. Further, the presence of more C-O type bonds in the
re
soluble lignin facilitates the hydrogenolysis of lignin into guaiacyl and syringyl monomeric
lP
units [177]. Want et al. studied the effect of different metal formate on the kraft lignin conversion into polyphenols. Around 23.7% (w/w) guaiacol was obtained from pure lignin and the production of polyphenols like different catechols was observed to significantly
na
increased with the addition of metal formats. A maximum yield of 22% of catechol was observed in the presence of nickel formate [178]. Incorporation of bronsted and lewis acid
ur
into the reaction medium was investigated to cleave C-O bonds present in lignin and
Jo
subsequently leads to produce monomeric units under mild acidic condition [179]. However, all such methods in presence or absence of hydrogen still faces a lot of challenges including corrosion, metal recovery to successfully implemented on an industrial basis. Apart from, in the presence of high temperature in the range of 300-1000 °C, lignin undergoes deformation into pyrolytic aromatic monomers such as mixture of phenols [180]. Likewise, exploration of the supercritical conditions of methanol or ethanol to yield catechol is another promising
46
method of producing phenolic compounds when alkylated lignin was used [181]. A number of different strategies involved in the conversion process of lignin is depicted in fig. 6. Fig. 6 (A) shows disintegration of lignin, which resulted in the ring openings and subsequent oxidation. Fig. 6(B) and (C) shows openings of the rings and conversion into carboxylic acids such as fumaric, maleic, muconic acids, succinic, malic and malonic acid. 5.2.2. Vanillin On a nut shell, vanillin has been the major form of phenol aldehydes industrially obtained
ro of
specifically from guaiacyl and syringyl units of lignin of lignocellulosic biomass [182]. It has been reported that, lignin obtained from sulphite pulping is much expensive compared to vanillin from guaiacol units [183]. During the production of vanillin, nitrobenzene was used
-p
as one of the staring reagent for oxidative depolymerization reaction of lignin [184]. But, toxic effect of nitrobenzene and separation problem with aniline as by product substituted
re
oxygen as alterative oxidant. Around 15% (w/w) mixture of vanillin and syringaldehyde was
5.2.3. Carboxylic acids
lP
obtained from steam exploded lignin in the presence of copper or ferrous ions [185].
During oxidation of linin, carboxylic acids are also produced as a side products with aromatic
na
aldehydes and phenols [186]. As lignin is the polymerized form of p-hydroxyphenyl, guaiacyl and syringyl subunits so, oxidation of lignin attibutes towards wide variety of carboxylic
ur
acids including aliphatic and aromatic with one or more carboxyl groups [187]. As these
Jo
acids are produced from fossil fuel based products so exploration of the renewable lignin has been a sustainable approach towards greener production of carboxylic acid. Now a days, research work is more focused on the conversion of aliphatic acid compared to aromatic acids as the market value of the aliphatic acids such as maleic acid and succinic acids are relatively high. Oxidation of lignin has been so effective process to disrupt the closed aromatic structure towards the subsequent formation of acids. In view of that, two different lignins
47
obtained from acid pretreated and steam pretreated biomass was oxidized using copper ferrous sulphide and in the presence of hydrogen peroxide, an average of 14% and 11% of dicarboxylic aliphatic acids are produced respectively, from two different lignins [188]. In the recent years the generation of hydroxyl radicals using Fenton’s reagent with H2O2 has been emerged as an effective oxidative media for lignin conversion into different aromatic acids [186, 189]. 5.3. Detoxification of inhibitors
ro of
As per as industrial acceptability is concerned, detoxification of fermentation inhibitors (originated from the conversion process of carbohydrate and lignin fraction of biomass) is of high importance. An effective detoxification method facilitates substantial elimination of
-p
inhibitors from the hydrolysate and reduce the impact of negative interference during the subsequent process of fermentation. Furfural generated from pentose sugars and
re
hydroxymethyl furfural obtained from hexose sugars of the biomass are detoxified after an
lP
evaporation of the hydrolysate with the aid of physical detoxification strategy [190]. In view of that, detoxification using activated carbon is an old and routine practice for an efficient removal of inhibitors without interfering the sugars present in the hydrolysate as mixture. In
na
order to prepare an inhibitor free feedstock for the fermentation obtained after acid hydrolysis of biomass, an addition of sodium hydroxide and calcium hydroxide into the reaction mixture
ur
influence the neutralization reaction for formic acid, acetic acid and levulinic acid present in
Jo
the hydrolysate [191]. Jeong et al. studied the effectiveness of complex extractant comprising a mixture of 20% trialkylamine, 70% of n-octanol and 10% of kerosene, which manifests towards 64% removal of furfural, acetic acid, formic acid and levulinic acid from the hydrolysate mixture [192]. Further, the presence of 1 g/L of lignin derivatives including – coumaric acid, ferulic acid, vanillin, syringaldehyde, 4-hydroxybenzoic acid inhibited more than 70% of cell growth in C. beijerinckii and completely paused butanol production while an
48
addition of 0.01 µmole of peroxidases enzyme significantly improved the cell growth and butanol production after detoxification of phenolic inhibitors [193]. Moreover, Zhang et al. observed 5% (w/v) of activated carbon effectively eliminated 78% of furan derivatives and 98.6% of aromatic monomers as inhibitors present in the poplar pre-hydrolyzate prior to the fermentation into biobutanol production.
6. Future perspective The entire aspect of lignocellulosic conversion is described with an effort to establish the
ro of
concept of valorization using biomass residues into value-added products. In view of that, a series of attempts were made to enumerate the effective processes based on their economic viability and demerits that can be used in tandem to produce value-added products from
-p
lignocellulosic feedstock. Hence, a number of different mechanical, physicochemical, chemical and biological pretreatment methods are comprehensively narrated as one of the
re
most critical and cost determining unit operations involved for an effective removal of the
lP
lignin component of the biomass. Primarily, mechanical operations, including ball milling and extrusion are observed with an ability to deconstruct the intricate structure of biomass at
na
an expense of mechanical and electrical energy. Whereas, physicochemical treatments like hot water, steam explosion and wet oxidation accounted for the rapid disruption in
ur
lignocellulosic structure with an involvement of high-pressure reactor. Further, chemical pretreatments are investigated to solubilize certain fraction of lignocellulosic components
Jo
when biomass is subjected to chemically treated with acid or base at certain reaction conditions. Acid hydrolysis is observed to solubilize maximum portion of hemicellulose and hydrolyze part of the cellulose of biomass in the prehydrolyzate along with the presence of few unwanted by-products known as inhibitors for fermentation process. As per as economic feasibility of the process is concerned, acid hydrolysis is hardly considered for large scale of industrial adaptation as the process of detoxification mandates significant elimination of the 49
toxic inhibitors prior to the fermentation process attributes towards an additional expense at the downstream processing. On the contrary, alkaline treatment using sodium hydroxide is considered as the most effective chemical pretreatment method as the process involves substantial delignification at an expense of mild reaction conditions. Based on the ground reality, other chemical treatments including ionic liquid and organic solvents necessitates the additional steps of recovery and reuse of the useful streams when economic acceptability of the process is concerned. Over the years, a number of various attempts are made in biological
ro of
pretreatment for an effective fractionation of biomass components with respect to time and process condition. However, the process of biological pretreatment demands subsequent opportunity to develop a rapid and cost-effective method for lignocellulosic biofuel
-p
production. On very brief note to say, the sole efficiency of any pretreatment process depends on the selection of the biomass based on the composition of cellulose, hemicellulose and
re
lignin. Hot water pretreatment may be an effective approach for the biomass with low lignin
lP
content as the process does not require the extraneous addition of any chemicals. On the contrary, the biomass with high content of lignin demands an efficient delignification process using mild alkaline reaction condition prior to enzymatic hydrolysis. The process of
na
enzymatic hydrolysis is mostly preferred as no co-formation of inhibitors are produced with monomeric sugars at the product side. Though enzymatic hydrolysis is considered as the most
ur
costly affair in the conversion process of biomass, but an application of integrated enzymes
Jo
from different sources attributes towards an increase in the production. However, the process cost invariably increases with the variation in enzyme application. Instead, the development of an enzyme with combined activity of cellulase, hemicellulase and ligninase encourages the feasibility of integrating the process of pretreatment and hydrolysis into a single cost effective step. During an initial phase of enzymatic hydrolysis, a rapid production of sugars is followed by a sudden declination in the sugars formation, which manifests towards the aspect
50
of product inhibition that interfere with the action of enzyme and process of hydrolysis. Following hydrolysis, another attempt was made to explore the hydrolytic and fermentation efficiency of lignocellulosic biomass for the production of bioethanol and biobutanol as biofuel. As per as fermentation is concerned, two subsequent processes of enzymatic hydrolysis and fermentation invariably increase the cost factor and make the process economically unsuitable. On the contrary, subsequent processes of hydrolysis and fermentation are simultaneously carried out together into a single reactor. However, there are
ro of
differences in the optimum reaction conditions of enzymatic hydrolysis and fermentation process. Therefore, both of the conventional modes of fermentation processes urge an indepth investigation by aiming at maximum yield using pretreated biomass as the feedstock
-p
material for biofuel production. Finally, various routes of producing the derivatives of cellulose and lignin into the valuable products directs the opportunity of exploring
re
lignocellulosic biomass as an indispensable option for biomass valorization.
lP
7. Conclusions
Apart from being a sustainable alternative, biofuel not only reduces the effect of greenhouse
na
gas emission, it also eases the much dependency on petroleum based conventional fuels. Like other global departments, The Government of India has already mandated a certain
ur
percentage of ethanol blending as a part of biofuel programme. Nevertheless, full-fledged commercialization of biofuel production still demands substantial improvements in the
Jo
existing technologies. Therefore, advancement in different pretreatment operations enable to decide on the selection of most cost economic option. Therefore, an exhaustive review on the readily available pretreatment methods along with the emerging technologies assist to build an overview of the initial unit operation. In view of that, pretreatment of biomass using hot water may be a promising approach as the process doesn’t demand any extraneous addition of chemicals to disrupt the structural intricacy of lignocellulosic residues. In the consequent 51
process, a detail understanding involved with the mechanistic interaction during enzymatic hydrolysis facilitates the production of fermentable sugars from pretreated biomass. With regard to the enzymatic system, hydrolysis of biomass in the presence of cellulase along with accessory enzymes excels the breakdown of β-(1-4) glycosidic bonds of polymeric carbohydrates for an effective saccharification process. However, as per as enzyme reusability is concerned, an efficiency of enzymatic process significantly depends on the cost of the enzyme. Further, an exploration of genetically modified microbes that primarily
ro of
possess the properties of cellulase, hemicellulase and cellobiase would have been most effective fermentation strategy during consolidated bioprocessing. Finally, an in depth knowledge of various biochemical routes involved in the transformation of cellulose and
-p
lignin into their derivatives critically analyzed the entire aspect of lignocellulosic conversion
re
process of biomass to biofuel to the readers.
lP
Disclosure statement
The authors declare that, there is no conflict of interest with the work for the present review
na
manuscript.
ur
Conflict of interest
The authors declare that, there is no conflict of interest with the work for the present review
Jo
manuscript (Ref No. PRBI_2019_908). Authors: Dibyajyoti Haldar, Mihir Kumar Purkait
52
Acknowledgement Centre for the Environment and R&D (Research and Development) section of IIT Guwahati is sincerely acknowledged for providing infrastructural facilities and institute funds through
Jo
ur
na
lP
re
-p
ro of
MHRD (Ministry of Human Resource and Development), Government of India.
53
References
Jo
ur
na
lP
re
-p
ro of
[1] D. Carrillo-Nieves, M.J. Rostro Alanís, R. de la Cruz Quiroz, H.A. Ruiz, H.M.N. Iqbal, R. Parra-Saldívar, Current status and future trends of bioethanol production from agro-industrial wastes in Mexico, Renewable Sustainable Energy Rev. 102 (2019) 63-74. [2] E. Hosseini Koupaie, S. Dahadha, A.A. Bazyar Lakeh, A. Azizi, E. Elbeshbishy, Enzymatic pretreatment of lignocellulosic biomass for enhanced biomethane production-A review, J. Environ. Manage. 233 (2019) 774-784. [3] H.M. Zabed, S. Akter, J. Yun, G. Zhang, F.N. Awad, X. Qi, J.N. Sahu, Recent advances in biological pretreatment of microalgae and lignocellulosic biomass for biofuel production, Renewable Sustainable Energy Rev. 105 (2019) 105-128. [4] D. Haldar, D. Sen, K. Gayen, A review on the production of fermentable sugars from lignocellulosic biomass through conventional and enzymatic route—a comparison, Int. J. Green Energy 13(12) (2016) 1232-1253. [5] K. Pandiyan, A. Singh, S. Singh, A.K. Saxena, L. Nain, Technological interventions for utilization of crop residues and weedy biomass for second generation bio-ethanol production, Renewable Energy 132 (2019) 723-741. [6] V.K. Ponnusamy, D.D. Nguyen, J. Dharmaraja, S. Shobana, J.R. Banu, R.G. Saratale, S.W. Chang, G. Kumar, A review on lignin structure, pretreatments, fermentation reactions and biorefinery potential, Bioresour. Technol. 271 (2019) 462-472. [7] S.Y. Balaman, Introduction to Biomass—Resources, Production, Harvesting, Collection, and Storage, in: S.Y. Balaman (Ed.), Decision-Making for Biomass-Based Production Chains, Academic Press2019, pp. 1-23. [8] S. Mohapatra, R.C. Ray, S. Ramachandran, Bioethanol from biorenewable feedstocks: Technology, economics, and challenges, in: R.C. Ray, S. Ramachandran (Eds.), Bioethanol Production from Food Crops, Academic Press2019, pp. 3-27. [9] A. Avanthi, S. Kumar, K.C. Sherpa, R. Banerjee, Bioconversion of hemicelluloses of lignocellulosic biomass to ethanol: An attempt to utilize pentose sugars, Biofuels 8(4) (2017) 431-444. [10] S.B. Jamaldheen, K. Sharma, A. Rani, V.S. Moholkar, A. Goyal, Comparative analysis of pretreatment methods on sorghum (Sorghum durra) stalk agrowaste for holocellulose content, Prep. Biochem. Biotechnol. 48(6) (2018) 457-464. [11] C. Nitsos, L. Matsakas, K. Triantafyllidis, U. Rova, P. Christakopoulos, Investigation of different pretreatment methods of mediterranean-type ecosystem agricultural residues: Characterisation of pretreatment products, high-solids enzymatic hydrolysis and bioethanol production, Biofuels 9(5) (2018) 545-558. [12] Hemansi, R. Gupta, G. Yadav, G. Kumar, A. Yadav, J.K. Saini, R.C. Kuhad, Second Generation bioethanol production: The state of art, in: N. Srivastava, M. Srivastava, P.K. Mishra, S.N. Upadhyay, P.W. Ramteke, V.K. Gupta (Eds.), Sustainable Approaches for Biofuels Production Technologies: From Current Status to Practical Implementation, Springer International Publishing, Cham, 2019, pp. 121-146. [13] J.K. Saini, R. Gupta, Hemansi, A. Verma, P. Gaur, R. Saini, R. Shukla, R.C. Kuhad, Integrated lignocellulosic biorefinery for sustainable bio-based economy, in: N. Srivastava, M. Srivastava, P.K. Mishra, S.N. Upadhyay, P.W. Ramteke, V.K. Gupta (Eds.), Sustainable Approaches for Biofuels Production Technologies: From Current Status to Practical Implementation, Springer International Publishing, Cham, 2019, pp. 25-46. [14] D. Kumari, R. Singh, Pretreatment of lignocellulosic wastes for biofuel production: A critical review, Renewable Sustainable Energy Rev. 90 (2018) 877-891. [15] S. Chen, X. Zhang, D. Singh, H. Yu, X. Yang, Biological pretreatment of lignocellulosics: Potential, progress and challenges, Biofuels 1(1) (2010) 177-199. 54
Jo
ur
na
lP
re
-p
ro of
[16] M. Mohan, N.N. Deshavath, T. Banerjee, V.V. Goud, V.V. Dasu, Ionic liquid and sulfuric acid-based pretreatment of bamboo: Biomass delignification and enzymatic hydrolysis for the production of reducing sugars, Ind. Eng. Chem. Res. 57(31) (2018) 1010510117. [17] N.N. Deshavath, S.K. Sahoo, M.M. Panda, S. Mahanta, D.S.N. Goutham, V.V. Goud, V.V. Dasu, A. Jetty, The cost-effective stirred tank reactor for cellulase production from alkaline-pretreated agriculture waste biomass, Springer Singapore, Singapore, 2018, pp. 2535. [18] A. Althuri, A.D. Chintagunta, K.C. Sherpa, R. Banerjee, Simultaneous saccharification and fermentation of lignocellulosic biomass, in: S. Kumar, R.K. Sani (Eds.), Biorefining of Biomass to Biofuels: Opportunities and Perception, Springer International Publishing, Cham, 2018, pp. 265-285. [19] I. Ntaikou, N. Menis, M. Alexandropoulou, G. Antonopoulou, G. Lyberatos, Valorization of kitchen biowaste for ethanol production via simultaneous saccharification and fermentation using co-cultures of the yeasts Saccharomyces cerevisiae and Pichia stipitis, Bioresour. Technol. 263 (2018) 75-83. [20] V. Soti, S. Lenaerts, I. Cornet, Of enzyme use in cost-effective high solid simultaneous saccharification and fermentation processes, J. Biotechnol. 270 (2018) 70-76. [21] D. Chen, A. Gao, K. Cen, J. Zhang, X. Cao, Z. Ma, Investigation of biomass torrefaction based on three major components: Hemicellulose, cellulose, and lignin, Energy Convers. Manage. 169 (2018) 228-237. [22] J.Y. Yeo, B.L.F. Chin, J.K. Tan, Y.S. Loh, Comparative studies on the pyrolysis of cellulose, hemicellulose, and lignin based on combined kinetics, J. Energy Inst. 92(1) (2019) 27-37. [23] H. Yu, Z. Wu, G. Chen, Catalytic gasification characteristics of cellulose, hemicellulose and lignin, Renewable Energy 121 (2018) 559-567. [24] X. Han, Y. Guo, X. Liu, Q. Xia, Y. Wang, Catalytic conversion of lignocellulosic biomass into hydrocarbons: A mini review, Catal. Today 319 (2019) 2-13. [25] X. Li, R. Xu, J. Yang, S. Nie, D. Liu, Y. Liu, C. Si, Production of 5hydroxymethylfurfural and levulinic acid from lignocellulosic biomass and catalytic upgradation, Ind. Crops Prod. 130 (2019) 184-197. [26] H. Wang, C. Zhu, D. Li, Q. Liu, J. Tan, C. Wang, C. Cai, L. Ma, Recent advances in catalytic conversion of biomass to 5-hydroxymethylfurfural and 2, 5-dimethylfuran, Renewable Sustainable Energy Rev. 103 (2019) 227-247. [27] N. Sarkar, S.K. Ghosh, S. Bannerjee, K. Aikat, Bioethanol production from agricultural wastes: An overview, Renewable Energy 37(1) (2012) 19-27. [28] H. Chen, J. Liu, X. Chang, D. Chen, Y. Xue, P. Liu, H. Lin, S. Han, A review on the pretreatment of lignocellulose for high-value chemicals, Fuel Processing Technology 160 (2017) 196-206. [29] P. Alvira, E. Tomás-Pejó, M. Ballesteros, M.J. Negro, Pretreatment technologies for an efficient bioethanol production process based on enzymatic hydrolysis: A review, Bioresource Technology 101(13) (2010) 4851-4861. [30] L. Yang, F. Xu, X. Ge, Y. Li, Challenges and strategies for solid-state anaerobic digestion of lignocellulosic biomass, Renewable and Sustainable Energy Reviews 44 (2015) 824-834. [31] M. Kumar, K. Gayen, Developments in biobutanol production: New insights, Applied Energy 88(6) (2011) 1999-2012. [32] A. Arevalo-Gallegos, Z. Ahmad, M. Asgher, R. Parra-Saldivar, H.M.N. Iqbal, Lignocellulose: A sustainable material to produce value-added products with a zero waste approach—A review, International Journal of Biological Macromolecules 99 (2017) 308-318. 55
Jo
ur
na
lP
re
-p
ro of
[33] M. Bilal, M. Asgher, H.M.N. Iqbal, H. Hu, X. Zhang, Biotransformation of lignocellulosic materials into value-added products—A review, International Journal of Biological Macromolecules 98 (2017) 447-458. [34] M. FitzPatrick, P. Champagne, M.F. Cunningham, R.A. Whitney, A biorefinery processing perspective: Treatment of lignocellulosic materials for the production of valueadded products, Bioresource Technology 101(23) (2010) 8915-8922. [35] S. Poundrik, National Policy on Biofuels, Ministry of Petroleum and Natural Gas Notification, New Delhi, 2018, pp. 13-23. [36] M. Raud, T. Kikas, O. Sippula, N.J. Shurpali, Potentials and challenges in lignocellulosic biofuel production technology, Renewable and Sustainable Energy Reviews 111 (2019) 44-56. [37] P. Phitsuwan, K. Sakka, K. Ratanakhanokchai, Improvement of lignocellulosic biomass in planta: A review of feedstocks, biomass recalcitrance, and strategic manipulation of ideal plants designed for ethanol production and processability, Biomass and Bioenergy 58 (2013) 390-405. [38] W. Tyner, The US ethanol and biofuels boom: Its origins, current status, and future prospects, BioScience 58 (2008) 646-653. [39] W. Tyner, Comparison of the US and EU approaches to stimulating biofuels, Biofuels 1 (2010) 19-21. [40] H. Chen, M.-l. Xu, Q. Guo, L. Yang, Y. Ma, A review on present situation and development of biofuels in China, Journal of the Energy Institute 89(2) (2016) 248-255. [41] S.D. Musa, T. Zhonghua, A.O. Ibrahim, M. Habib, China's energy status: A critical look at fossils and renewable options, Renewable and Sustainable Energy Reviews 81 (2018) 2281-2290. [42] L. Kratky, T. Jirout, Biomass size reduction machines for enhancing biogas production, Chem. Eng. Technol. 34(3) (2011) 391-399. [43] G.G. Silva, M. Couturier, J.G. Berrin, A. Buleon, X. Rouau, Effects of grinding processes on enzymatic degradation of wheat straw, Bioresour Technol 103(1) (2012) 192200. [44] A. Barakat, C. Mayer-Laigle, A. Solhy, R.A.D. Arancon, H. de Vries, R. Luque, Mechanical pretreatments of lignocellulosic biomass: Towards facile and environmentally sound technologies for biofuels production, RSC Adv. 4(89) (2014) 48109-48127. [45] M. Lu, J. Li, L. Han, W. Xiao, An aggregated understanding of cellulase adsorption and hydrolysis for ball-milled cellulose, Bioresour Technol 273 (2019) 1-7. [46] Y.M. Gu, H. Kim, B.-I. Sang, J.H. Lee, Effects of water content on ball milling pretreatment and the enzymatic digestibility of corn stover, Water-Energy Nexus 1(1) (2018) 61-65. [47] Z. Yuan, J. Long, T. Wang, R. Shu, Q. Zhang, L. Ma, Process intensification effect of ball milling on the hydrothermal pretreatment for corn straw enzymolysis, Energy Convers. Manage. 101 (2015) 481-488. [48] S. Senturk-Ozer, H. Gevgilili, D.M. Kalyon, Biomass pretreatment strategies via control of rheological behavior of biomass suspensions and reactive twin screw extrusion processing, Bioresour. Technol. 102(19) (2011) 9068-9075. [49] A.S.A. da Silva, R.S.S. Teixeira, T. Endo, E.P.S. Bon, S.-H. Lee, Continuous pretreatment of sugarcane bagasse at high loading in an ionic liquid using a twin-screw extruder, Green Chem. 15(7) (2013) 1991-2001. [50] B. Lamsal, J. Yoo, K. Brijwani, S. Alavi, Extrusion as a thermo-mechanical pretreatment for lignocellulosic ethanol, Biomass Bioenergy 34(12) (2010) 1703-1710.
56
Jo
ur
na
lP
re
-p
ro of
[51] C. Karunanithy, K. Muthukumarappan, Effect of extruder parameters and moisture content of switchgrass, prairie cord grass on sugar recovery from enzymatic hydrolysis, Applied biochemistry and biotechnology 162(6) (2010) 1785-803. [52] M. Han, K.E. Kang, Y. Kim, G.-W. Choi, High efficiency bioethanol production from barley straw using a continuous pretreatment reactor, Process Biochem. 48(3) (2013) 488495. [53] Z. Lin, H. Huang, H. Zhang, L. Zhang, L. Yan, J. Chen, Ball milling pretreatment of corn stover for enhancing the efficiency of enzymatic hydrolysis, Appl. Biochem. Biotechnol. 162(7) (2010) 1872-1880. [54] Y. Sewsynker-Sukai, E.B. Gueguim Kana, Microwave-assisted alkalic salt pretreatment of corn cob wastes: Process optimization for improved sugar recovery, Ind. Crops Prod. 125 (2018) 284-292. [55] P.B. Subhedar, P. Ray, P.R. Gogate, Intensification of delignification and subsequent hydrolysis for the fermentable sugar production from lignocellulosic biomass using ultrasonic irradiation, Ultrason. Sonochem. 40 (2018) 140-150. [56] S. Tsubaki, J.-i. Azuma, S. Fujii, R. Singh, B. Thallada, Y. Wada, Microwave-driven biorefinery for utilization of food and agricultural waste biomass, in: T. Bhaskar, A. Pandey, S.V. Mohan, D.-J. Lee, S.K. Khanal (Eds.), Waste Biorefinery, Elsevier2018, pp. 393-408. [57] B.-M. Lee, J.-P. Jeun, P.-H. Kang, Enhanced enzymatic hydrolysis of kenaf core using irradiation and dilute acid, Radiat. Phys. Chem. 130 (2017) 216-220. [58] Y. Yin, J. Wang, Enhancement of enzymatic hydrolysis of wheat straw by gamma irradiation–alkaline pretreatment, Radiat. Phys. Chem. 123 (2016) 63-67. [59] K. Kapoor, N. Garg, R.K. Diwan, L. Varshney, A.K. Tyagi, Study the effect of gamma radiation pretreatment of sugarcane bagasse on its physcio-chemical morphological and structural properties, Radiat. Phys. Chem. 141 (2017) 190-195. [60] R. Liu, W. Tian, S. Kong, Y. Meng, H. Wang, J. Zhang, Effects of inorganic and organic acid pretreatments on the hydrothermal liquefaction of municipal secondary sludge, Energy Convers. Manage. 174 (2018) 661-667. [61] A. Mishra, S. Ghosh, Bioethanol production from various lignocellulosic feedstocks by a novel “fractional hydrolysis” technique with different inorganic acids and co-culture fermentation, Fuel 236 (2019) 544-553. [62] C.K. Nitsos, T. Choli-Papadopoulou, K.A. Matis, K.S. Triantafyllidis, Optimization of hydrothermal pretreatment of hardwood and softwood lignocellulosic residues for selective hemicellulose recovery and improved cellulose enzymatic hydrolysis, ACS Sustainable Chem. Eng. 4(9) (2016) 4529-4544. [63] Z. Yu, Y. Du, X. Shang, Y. Zheng, J. Zhou, Enhancing fermentable sugar yield from cassava residue using a two-step dilute ultra-low acid pretreatment process, Ind. Crops Prod. 124 (2018) 555-562. [64] Q. Liu, W. Li, Q. Ma, S. An, M. Li, H. Jameel, H.M. Chang, Pretreatment of corn stover for sugar production using a two-stage dilute acid followed by wet-milling pretreatment process, Bioresour Technol 211 (2016) 435-42. [65] S. An, W. Li, Q. Liu, Y. Xia, T. Zhang, F. Huang, Q. Lin, L. Chen, Combined dilute hydrochloric acid and alkaline wet oxidation pretreatment to improve sugar recovery of corn stover, Bioresour. Technol. 271 (2019) 283-288. [66] S. Chen, Z. Ling, X. Zhang, Y.S. Kim, F. Xu, Towards a multi-scale understanding of dilute hydrochloric acid and mild 1-ethyl-3-methylimidazolium acetate pretreatment for improving enzymatic hydrolysis of poplar wood, Ind. Crops Prod. 114 (2018) 123-131. [67] K. Rattanaporn, S. Roddecha, M. Sriariyanun, K. Cheenkachorn, Improving saccharification of oil palm shell by acetic acid pretreatment for biofuel production, Energy Procedia 141 (2017) 146-149. 57
Jo
ur
na
lP
re
-p
ro of
[68] P. Wang, Y.M. Chen, Y. Wang, Y.Y. Lee, W. Zong, S. Taylor, T. McDonald, Y. Wang, Towards comprehensive lignocellulosic biomass utilization for bioenergy production: Efficient biobutanol production from acetic acid pretreated switchgrass with Clostridium saccharoperbutylacetonicum N1-4, Appl. Energy 236 (2019) 551-559. [69] O.-M. Kwon, D.-H. Kim, S.-K. Kim, G.-T. Jeong, Production of sugars from macroalgae Gracilaria verrucosa using combined process of citric acid-catalyzed pretreatment and enzymatic hydrolysis, Algal Research 13 (2016) 293-297. [70] S.-Y. Jeong, J.-W. Lee, Optimization of pretreatment condition for ethanol production from oxalic acid pretreated biomass by response surface methodology, Ind. Crops Prod. 79 (2016) 1-6. [71] D. Haldar, D. Sen, K. Gayen, Development of spectrophotometric method for the analysis of multi-component carbohydrate mixture of different moieties, Appl. Biochem. Biotechnol. 181(4) (2017) 1416-1434. [72] H. Yang, Z. Shi, G. Xu, Y. Qin, J. Deng, J. Yang, Bioethanol production from bamboo with alkali-catalyzed liquid hot water pretreatment, Bioresour. Technol. 274 (2019) 261-266. [73] H. Li, X. Chen, L. Xiong, M. Luo, X. Chen, C. Wang, C. Huang, X. Chen, Stepwise enzymatic hydrolysis of alkaline oxidation treated sugarcane bagasse for the co-production of functional xylo-oligosaccharides and fermentable sugars, Bioresour. Technol. 275 (2019) 345-351. [74] M. Lainez, H.A. Ruiz, A.A. Castro-Luna, S. Martínez-Hernández, Release of simple sugars from lignocellulosic biomass of Agave salmiana leaves subject to sequential pretreatment and enzymatic saccharification, Biomass Bioenergy 118 (2018) 133-140. [75] K. Shill, S. Padmanabhan, Q. Xin, J.M. Prausnitz, D.S. Clark, H.W. Blanch, Ionic liquid pretreatment of cellulosic biomass: Enzymatic hydrolysis and ionic liquid recycle, Biotechnol. Bioeng. 108(3) (2011) 511-20. [76] Y. Fukaya, A. Sugimoto, H. Ohno, Superior solubility of polysaccharides in low viscosity, polar, and halogen-free 1,3-dialkylimidazolium formates, Biomacromolecules 7(12) (2006) 3295-7. [77] E.S. Sashina, D.A. Kashirskii, M. Zaborski, S. Jankowski, Synthesis and dissolving power of 1-Alkyl-3-methylpyridinium-based ionic liquids, Russ. J. Gen. Chem. 82(12) (2012) 1994-1998. [78] S. Saher, H. Saleem, A.M. Asim, M. Uroos, N. Muhammad, Pyridinium based ionic liquid: A pretreatment solvent and reaction medium for catalytic conversion of cellulose to total reducing sugars (TRS), J. Mol. Liq. 272 (2018) 330-336. [79] L.T.P. Trinh, Y.-J. Lee, C.S. Park, H.-J. Bae, Aqueous acidified ionic liquid pretreatment for bioethanol production and concentration of produced ethanol by pervaporation, J. Ind. Eng. Chem. 69 (2019) 57-65. [80] R.d.C.M. Miranda, J.V. Neta, L.F. Romanholo Ferreira, W.A. Gomes, C.S. do Nascimento, E. de B. Gomes, S. Mattedi, C.M.F. Soares, Á.S. Lima, Pineapple crown delignification using low-cost ionic liquid based on ethanolamine and organic acids, Carbohydr. Polym. 206 (2019) 302-308. [81] M.R. Sturgeon, S. Kim, K. Lawrence, R.S. Paton, S.C. Chmely, M. Nimlos, T.D. Foust, G.T. Beckham, A mechanistic investigation of acid-catalyzed cleavage of aryl-ether linkages: Implications for lignin depolymerization in acidic environments, ACS Sustainable Chem. Eng. 2(3) (2014) 472-485. [82] A. Satlewal, R. Agrawal, S. Bhagia, J. Sangoro, A.J. Ragauskas, Natural deep eutectic solvents for lignocellulosic biomass pretreatment: Recent developments, challenges and novel opportunities, Biotechnol. Adv. 36(8) (2018) 2032-2050. [83] E.K. New, T.Y. Wu, C.B. Tien Loong Lee, Z.Y. Poon, Y.-L. Loow, L.Y. Wei Foo, A. Procentese, L.F. Siow, W.H. Teoh, N.N. Nik Daud, J.M. Jahim, A.W. Mohammad, Potential 58
Jo
ur
na
lP
re
-p
ro of
use of pure and diluted choline chloride-based deep eutectic solvent in delignification of oil palm fronds, Process Saf. Environ. Prot. 123 (2019) 190-198. [84] Q. Yu, L. Qin, Y. Liu, Y. Sun, H. Xu, Z. Wang, Z. Yuan, In situ deep eutectic solvent pretreatment to improve lignin removal from garden wastes and enhance production of biomethane and microbial lipids, Bioresour. Technol. 271 (2019) 210-217. [85] Z. Chen, W.A. Jacoby, C. Wan, Ternary deep eutectic solvents for effective biomass deconstruction at high solids and low enzyme loadings, Bioresour. Technol. (2019). [86] A. Duque, P. Manzanares, I. Ballesteros, M. Ballesteros, Steam explosion as lignocellulosic biomass pretreatment, in: S.I. Mussatto (Ed.), Biomass Fractionation Technologies for a Lignocellulosic Feedstock Based Biorefinery, Elsevier, Amsterdam, 2016, pp. 349-368. [87] F.M.V. Oliveira, I.O. Pinheiro, A.M. Souto-Maior, C. Martin, A.R. Gonçalves, G.J.M. Rocha, Industrial-scale steam explosion pretreatment of sugarcane straw for enzymatic hydrolysis of cellulose for production of second generation ethanol and value-added products, Bioresour. Technol. 130 (2013) 168-173. [88] A. Verardi, A. Blasi, T. Marino, A. Molino, V. Calabrò, Effect of steam-pretreatment combined with hydrogen peroxide on lignocellulosic agricultural wastes for bioethanol production: Analysis of derived sugars and other by-products, J Energy Chem. 27(2) (2018) 535-543. [89] R. Kataria, A. Mol, E. Schulten, A. Happel, S.I. Mussatto, Bench scale steam explosion pretreatment of acid impregnated elephant grass biomass and its impacts on biomass composition, structure and hydrolysis, Ind. Crops Prod. 106 (2017) 48-58. [90] Z. Wang, G. Wu, L.J. Jönsson, Effects of impregnation of softwood with sulfuric acid and sulfur dioxide on chemical and physical characteristics, enzymatic digestibility, and fermentability, Bioresour. Technol. 247 (2018) 200-208. [91] G. Batista, R.B.A. Souza, B. Pratto, M.S.R. dos Santos-Rocha, A.J.G. Cruz, Effect of severity factor on the hydrothermal pretreatment of sugarcane straw, Bioresour. Technol. 275 (2019) 321-327. [92] D. Haldar, D. Sen, K. Gayen, Enzymatic hydrolysis of banana stems (Musa acuminata): Optimization of process parameters and inhibition characterization, Int. J. Green Energy 15(6) (2018) 406-413. [93] H. Lyu, J. Zhou, Z. Geng, C. Lyu, Y. Li, Two-stage processing of liquid hot water pretreatment for recovering C5 and C6 sugars from cassava straw, Process Biochem. 75 (2018) 202-211. [94] R. Biswas, H. Uellendahl, B.K. Ahring, Wet Explosion: a Universal and Efficient Pretreatment Process for Lignocellulosic Biorefineries, Bioenergy Res. 8(3) (2015) 11011116. [95] M. Balat, H. Balat, C. Öz, Progress in bioethanol processing, Prog. Energy Combust. Sci. 34(5) (2008) 551-573. [96] S.S. Toor, L. Rosendahl, A. Rudolf, Hydrothermal liquefaction of biomass: A review of subcritical water technologies, Energy 36(5) (2011) 2328-2342. [97] D. Deswal, R. Gupta, P. Nandal, R.C. Kuhad, Fungal pretreatment improves amenability of lignocellulosic material for its saccharification to sugars, Carbohydr. Polym. 99 (2014) 264-269. [98] J.C. Martínez-Patiño, T.A. Lu-Chau, B. Gullón, E. Ruiz, I. Romero, E. Castro, J.M. Lema, Application of a combined fungal and diluted acid pretreatment on olive tree biomass, Ind. Crops. Prod. 121 (2018) 10-17. [99] A.K. Kumar, S. Sharma, Recent updates on different methods of pretreatment of lignocellulosic feedstocks: a review, Bioresources and bioprocessing 4(1) (2017) 7.
59
Jo
ur
na
lP
re
-p
ro of
[100] R. Ravindran, S. Jaiswal, N. Abu-Ghannam, A.K. Jaiswal, A comparative analysis of pretreatment strategies on the properties and hydrolysis of brewers’ spent grain, Bioresource Technology 248 (2018) 272-279. [101] S.J. Yang, H.Y. Yoo, H.S. Choi, J.H. Lee, C. Park, S.W. Kim, Enhancement of enzymatic digestibility of Miscanthus by electron beam irradiation and chemical combined treatments for bioethanol production, Chemical Engineering Journal 275 (2015) 227-234. [102] P. Kumar, D.M. Barrett, M.J. Delwiche, P. Stroeve, Pulsed Electric Field Pretreatment of Switchgrass and Wood Chip Species for Biofuel Production, Industrial & Engineering Chemistry Research 50(19) (2011) 10996-11001. [103] S.C.T. Oliveira, A.B. Figueiredo, D.V. Evtuguin, J.A. Saraiva, High pressure treatment as a tool for engineering of enzymatic reactions in cellulosic fibres, Bioresource Technology 107 (2012) 530-534. [104] M. Raud, J. Olt, T. Kikas, N2 explosive decompression pretreatment of biomass for lignocellulosic ethanol production, Biomass and Bioenergy 90 (2016) 1-6. [105] R. Ravindran, C. Sarangapani, S. Jaiswal, P.J. Cullen, A.K. Jaiswal, Ferric chloride assisted plasma pretreatment of lignocellulose, Bioresour Technol 243 (2017) 327-334. [106] Q. Qing, C.E. Wyman, Supplementation with xylanase and beta-xylosidase to reduce xylo-oligomer and xylan inhibition of enzymatic hydrolysis of cellulose and pretreated corn stover, Biotechnol Biofuels 4(1) (2011) 18. [107] A.A. Modenbach, S.E. Nokes, Enzymatic hydrolysis of biomass at high-solids loadings – A review, Biomass Bioenergy 56 (2013) 526-544. [108] H. Fang, L. Xia, Cellulase production by recombinant Trichoderma reesei and its application in enzymatic hydrolysis of agricultural residues, Fuel 143 (2015) 211-216. [109] D. Haldar, K. Gayen, D. Sen, Enumeration of monosugars’ inhibition characteristics on the kinetics of enzymatic hydrolysis of cellulose, Process Biochem. 72 (2018) 130-136. [110] Q.-S. Meng, C.-G. Liu, X.-Q. Zhao, F.-W. Bai, Engineering Trichoderma reesei RutC30 with the overexpression of egl1 at the ace1 locus to relieve repression on cellulase production and to adjust the ratio of cellulolytic enzymes for more efficient hydrolysis of lignocellulosic biomass, J. Biotechnol. 285 (2018) 56-63. [111] C.-H. Chen, J.-Y. Yao, B. Yang, H.-L. Lee, S.-F. Yuan, H.-Y. Hsieh, P.-H. Liang, Engineer multi-functional cellulase/xylanase/β-glucosidase with improved efficacy to degrade rice straw, Bioresour. Technol. Rep. 5 (2019) 170-177. [112] Y. Xia, L. Yang, L. Xia, High-level production of a fungal β-glucosidase with application potentials in the cost-effective production of Trichoderma reesei cellulase, Process Biochem. 70 (2018) 55-60. [113] A. Boyce, G. Walsh, Characterisation of a novel thermostable endoglucanase from Alicyclobacillus vulcanalis of potential application in bioethanol production, Appl. Microbiol. Biotechnol. 99(18) (2015) 7515-25. [114] J. Rocha-Martín, C. Martinez-Bernal, Y. Pérez-Cobas, F.M. Reyes-Sosa, B.D. García, Additives enhancing enzymatic hydrolysis of lignocellulosic biomass, Bioresour. Technol. 244 (2017) 48-56. [115] S.D. Mansfield, C. Mooney, J.N. Saddler, Substrate and Enzyme Characteristics that Limit Cellulose Hydrolysis, Biotechnology Progress 15(5) (1999) 804-816. [116] A.R. Esteghlalian, V. Srivastava, N. Gilkes, D.J. Gregg, J.N. Saddler, An Overview of Factors Influencing the Enzymatic Hydrolysis of Lignocellulosic Feedstocks, Glycosyl Hydrolases for Biomass Conversion, American Chemical Society2000, pp. 100-111. [117] X. Pan, D. Xie, N. Gilkes, D.J. Gregg, J.N. Saddler, Strategies to enhance the enzymatic hydrolysis of pretreated softwood with high residual lignin content, Applied biochemistry and biotechnology 124(1) (2005) 1069.
60
Jo
ur
na
lP
re
-p
ro of
[118] A. Asztalos, M. Daniels, A. Sethi, T. Shen, P. Langan, A. Redondo, S. Gnanakaran, A coarse-grained model for synergistic action of multiple enzymes on cellulose, Biotechnol. Biofuels 5(1) (2012) 55. [119] E. Hoshino, M. Shiroishi, Y. Amano, M. Nomura, T. Kanda, Synergistic actions of exo-type cellulases in the hydrolysis of cellulose with different crystallinities, J. Ferment. Bioeng. 84(4) (1997) 300-306. [120] Y. Zhao, B. Wu, B. Yan, P. Gao, Mechanism of cellobiose inhibition in cellulose hydrolysis by cellobiohydrolase, Sci China C Life Sci. 47(1) (2004) 18-24. [121] J. Li, L. Du, L. Wang, Glycosidic-bond hydrolysis mechanism catalyzed by cellulase Cel7A from Trichoderma reesei: A comprehensive theoretical study by performing MD, QM, and QM/MM calculations, The journal of physical chemistry. B 114(46) (2010) 15261-8. [122] V. Prévot, M. Lopez, E. Copinet, F. Duchiron, Comparative performance of commercial and laboratory enzymatic complexes from submerged or solid-state fermentation in lignocellulosic biomass hydrolysis, Bioresource Technology 129 (2013) 690-693. [123] M. Kirupa Sankar, R. Ravikumar, M. Naresh Kumar, U. Sivakumar, Development of co-immobilized tri-enzyme biocatalytic system for one-pot pretreatment of four different perennial lignocellulosic biomass and evaluation of their bioethanol production potential, Bioresource Technology 269 (2018) 227-236. [124] J. Börjesson, M. Engqvist, B. Sipos, F. Tjerneld, Effect of poly(ethylene glycol) on enzymatic hydrolysis and adsorption of cellulase enzymes to pretreated lignocellulose, Enzyme and Microbial Technology 41(1) (2007) 186-195. [125] D. Dahnum, S.O. Tasum, E. Triwahyuni, M. Nurdin, H. Abimanyu, Comparison of SHF and SSF processes using enzyme and dry yeast for optimization of bioethanol production from empty fruit bunch, Energy Procedia 68 (2015) 107-116. [126] W.R.d.S. Trindade, R.G.d. Santos, Review on the characteristics of butanol, its production and use as fuel in internal combustion engines, Renewable Sustainable Energy Rev. 69 (2017) 642-651. [127] K. Charubin, R.K. Bennett, A.G. Fast, E.T. Papoutsakis, Engineering Clostridium organisms as microbial cell-factories: challenges & opportunities, Metab. Eng. 50 (2018) 173-191. [128] P. Majidian, M. Tabatabaei, M. Zeinolabedini, M.P. Naghshbandi, Y. Chisti, Metabolic engineering of microorganisms for biofuel production, Renewable Sustainable Energy Rev. 82 (2018) 3863-3885. [129] N. Singh, A.S. Mathur, R.P. Gupta, C.J. Barrow, D. Tuli, M. Puri, Enhanced cellulosic ethanol production via consolidated bioprocessing by Clostridium thermocellum ATCC 31924☆, Bioresource Technology 250 (2018) 860-867. [130] F. Xin, W. Dong, W. Zhang, J. Ma, M. Jiang, Biobutanol production from crystalline cellulose through consolidated bioprocessing, Trends Biotechnol. 37(2) (2019) 167-180. [131] Y. Jiang, D. Guo, J. Lu, P. Dürre, W. Dong, W. Yan, W. Zhang, J. Ma, M. Jiang, F. Xin, Consolidated bioprocessing of butanol production from xylan by a thermophilic and butanologenic Thermoanaerobacterium sp. M5, Biotechnol. Biofuels 11(1) (2018) 89. [132] S. Brethauer, M.H. Studer, Consolidated bioprocessing of lignocellulose by a microbial consortium, Energy Environ. Sci. 7(4) (2014) 1446-1453. [133] L. Liu, J. Yang, Y. Yang, L. Luo, R. Wang, Y. Zhang, H. Yuan, Consolidated bioprocessing performance of bacterial consortium EMSD5 on hemicellulose for isopropanol production, Bioresource Technology 292 (2019) 121965. [134] L. Liu, Z. Zhang, J. Wang, Y. Fan, W. Shi, X. Liu, Q. Shun, Simultaneous saccharification and co-fermentation of corn stover pretreated by H2O2 oxidative degradation for ethanol production, Energy 168 (2019) 946-952.
61
Jo
ur
na
lP
re
-p
ro of
[135] L.M. Nieves, L.A. Panyon, X. Wang, Engineering Sugar Utilization and Microbial Tolerance toward Lignocellulose Conversion, Frontiers in bioengineering and biotechnology 3 (2015) 17. [136] M.T. Fernández-Sandoval, J. Galíndez-Mayer, F. Bolívar, G. Gosset, O.T. Ramírez, A. Martinez, Xylose–glucose co-fermentation to ethanol by Escherichia coli strain MS04 using single- and two-stage continuous cultures under micro-aerated conditions, Microbial cell factories 18(1) (2019) 145. [137] R. Yao, K. Shimizu, Recent progress in metabolic engineering for the production of biofuels and biochemicals from renewable sources with particular emphasis on catabolite regulation and its modulation, Process Biochemistry 48(9) (2013) 1409-1417. [138] T. Li, J. He, Simultaneous saccharification and fermentation of hemicellulose to butanol by a non-sporulating Clostridium species, Bioresour. Technol. 219 (2016) 430-438. [139] K. Ma, Z. Ruan, Production of a lignocellulolytic enzyme system for simultaneous biodelignification and saccharification of corn stover employing co-culture of fungi, Bioresour. Technol. 175 (2015) 586-93. [140] M. Karimi, R. Esfandiar, D. Biria, Simultaneous delignification and saccharification of rice straw as a lignocellulosic biomass by immobilized Thrichoderma viride sp. to enhance enzymatic sugar production, Renewable Energy 104 (2017) 88-95. [141] Y.-G. Lee, Y.-S. Jin, Y.-L. Cha, J.-H. Seo, Bioethanol production from cellulosic hydrolysates by engineered industrial Saccharomyces cerevisiae, Bioresour. Technol. 228 (2017) 355-361. [142] S.H. Mohd Azhar, R. Abdulla, Bioethanol production from galactose by immobilized wild-type Saccharomyces cerevisiae, Biocatal. Agric. Biotechnol. 14 (2018) 457-465. [143] D.T.T. Nguyen, P. Praveen, K.-C. Loh, Zymomonas mobilis immobilization in polymeric membranes for improved resistance to lignocellulose-derived inhibitors in bioethanol fermentation, Biochem. Eng. J. 140 (2018) 29-37. [144] Q. Zhang, Nurhayati, C.-L. Cheng, Y.-C. Lo, D. Nagarajan, J. Hu, J.-S. Chang, D.-J. Lee, Ethanol production by modified polyvinyl alcohol-immobilized Zymomonas mobilis and in situ membrane distillation under very high gravity condition, Appl. Energy 202 (2017) 1-5. [145] R.M. Ferreira, M.J. Mota, R.P. Lopes, S. Sousa, A.M. Gomes, I. Delgadillo, J.A. Saraiva, Adaptation of Saccharomyces cerevisiae to high pressure (15, 25 and 35 MPa) to enhance the production of bioethanol, Food Res. Int. 115 (2019) 352-359. [146] P.D. Nagy, Exploitation of a surrogate host, Saccharomyces cerevisiae, to identify cellular targets and develop novel antiviral approaches, Curr. Opin. Virol. 26 (2017) 132-140. [147] X.H. Hu, M.H. Wang, T. Tan, J.R. Li, H. Yang, L. Leach, R.M. Zhang, Z.W. Luo, Genetic dissection of ethanol tolerance in the budding yeast Saccharomyces cerevisiae, Genetics 175(3) (2007) 1479-87. [148] J.E. DiCarlo, J.E. Norville, P. Mali, X. Rios, J. Aach, G.M. Church, Genome engineering in Saccharomyces cerevisiae using CRISPR-Cas systems, Nucleic Acids Res. 41(7) (2013) 4336-43. [149] M. Bertilsson, J. Andersson, G. Liden, Modeling simultaneous glucose and xylose uptake in Saccharomyces cerevisiae from kinetics and gene expression of sugar transporters, Bioprocess Biosyst. Eng. 31(4) (2008) 369-77. [150] E. Young, A. Poucher, A. Comer, A. Bailey, H. Alper, Functional survey for heterologous sugar transport proteins, using Saccharomyces cerevisiae as a Host, Appl. Environ. Microbiol. 77(10) (2011) 3311-3319. [151] S. Lane, H. Xu, E.J. Oh, H. Kim, A. Lesmana, D. Jeong, G. Zhang, C.-S. Tsai, Y.-S. Jin, S.R. Kim, Glucose repression can be alleviated by reducing glucose phosphorylation rate in Saccharomyces cerevisiae, Sci. Rep. 8(1) (2018) 2613.
62
Jo
ur
na
lP
re
-p
ro of
[152] A. Farwick, S. Bruder, V. Schadeweg, M. Oreb, E. Boles, Engineering of yeast hexose transporters to transport D-xylose without inhibition by D-glucose, Proceedings of the National Academy of Sciences of the United States of America 111(14) (2014) 5159-5164. [153] A. Ajit, A.Z. Sulaiman, Y. Chisti, Production of bioethanol by Zymomonas mobilis in high-gravity extractive fermentations, Food Bioprod. Process. 102 (2017) 123-135. [154] Y. You, S. Liu, B. Wu, Y.-W. Wang, Q.-L. Zhu, H. Qin, F.-R. Tan, Z.-Y. Ruan, K.-D. Ma, L.-C. Dai, M. Zhang, G.-Q. Hu, M.-X. He, Bio-ethanol production by Zymomonas mobilis using pretreated dairy manure as a carbon and nitrogen source, RSC Adv. 7(7) (2017) 3768-3779. [155] S. Yang, Q. Fei, Y. Zhang, L.M. Contreras, S.M. Utturkar, S.D. Brown, M.E. Himmel, M. Zhang, Zymomonas mobilis as a model system for production of biofuels and biochemicals, Microb. Biotechnol. 9(6) (2016) 699-717. [156] K. Deanda, M. Zhang, C. Eddy, S. Picataggio, Development of an arabinosefermenting Zymomonas mobilis strain by metabolic pathway engineering, Appl. Environ. Microbiol. 62(12) (1996) 4465-70. [157] C. Xue, J. Zhao, L. Chen, S.-T. Yang, F. Bai, Recent advances and state-of-the-art strategies in strain and process engineering for biobutanol production by Clostridium acetobutylicum, Biotechnol. Adv. 35(2) (2017) 310-322. [158] C. Ren, Y. Gu, S. Hu, Y. Wu, P. Wang, Y. Yang, C. Yang, S. Yang, W. Jiang, Identification and inactivation of pleiotropic regulator CcpA to eliminate glucose repression of xylose utilization in Clostridium acetobutylicum, Metab. Eng. 12(5) (2010) 446-54. [159] G. De Bhowmick, A.K. Sarmah, R. Sen, Lignocellulosic biorefinery as a model for sustainable development of biofuels and value added products, Bioresour. Technol. 247 (2018) 1144-1154. [160] J. Song, H. Fan, J. Ma, B. Han, Conversion of glucose and cellulose into value-added products in water and ionic liquids, Green Chem. 15(10) (2013) 2619-2635. [161] S. Zhang, O. Metin, D. Su, S. Sun, Monodisperse AgPd alloy nanoparticles and their superior catalysis for the dehydrogenation of formic acid, Angew. Chem. Int. Ed. Engl. 52(13) (2013) 3681-4. [162] J. Yun, F. Jin, A. Kishita, K. Tohji, H. Enomoto, Formic acid production from carbohydrates biomass by hydrothermal reaction, J. Phys. Conf. Ser. 215 (2010) 012126. [163] W. Wang, M. Niu, Y. Hou, W. Wu, Z. Liu, Q. Liu, S. Ren, K.N. Marsh, Catalytic conversion of biomass-derived carbohydrates to formic acid using molecular oxygen, Green Chem. 16(5) (2014) 2614-2618. [164] P. Gao, G. Li, F. Yang, X.-N. Lv, H. Fan, L. Meng, X.-Q. Yu, Preparation of lactic acid, formic acid and acetic acid from cotton cellulose by the alkaline pre-treatment and hydrothermal degradation, Ind. Crops Prod. 48 (2013) 61-67. [165] F. Jin, Z. Zhou, T. Moriya, H. Kishida, H. Higashijima, H. Enomoto, Controlling hydrothermal reaction pathways to improve acetic acid production from carbohydrate biomass, Environ. Sci. Technol. 39(6) (2005) 1893-1902. [166] F. Jin, Z. Zhou, A. Kishita, H. Enomoto, H. Kishida, T. Moriya, A New hydrothermal process for producing acetic acid from biomass waste, Chem. Eng. Res. Des. 85(2) (2007) 201-206. [167] Z. Huo, Y. Fang, G. Yao, X. Zeng, D. Ren, F. Jin, Improved two-step hydrothermal process for acetic acid production from carbohydrate biomass, J. Energy Chem. 24(2) (2015) 207-212. [168] I. Obregón, I. Gandarias, M.G. Al-Shaal, C. Mevissen, P.L. Arias, R. Palkovits, The role of the hydrogen source on the selective production of γ-Valerolactone and 2Methyltetrahydrofuran from levulinic acid, ChemSusChem 9(17) (2016) 2488-2495.
63
Jo
ur
na
lP
re
-p
ro of
[169] Z. Sun, L. Xue, S. Wang, X. Wang, J. Shi, Single step conversion of cellulose to levulinic acid using temperature-responsive dodeca-aluminotungstic acid catalysts, Green Chem. 18(3) (2016) 742-752. [170] H. Jeong, S.-Y. Park, G.-H. Ryu, J.-H. Choi, J.-H. Kim, W.-S. Choi, S.M. Lee, J.W. Choi, I.-G. Choi, Catalytic conversion of hemicellulosic sugars derived from biomass to levulinic acid, Catal. Commun. 117 (2018) 19-25. [171] A.S. Khan, Z. Man, M.A. Bustam, A. Nasrullah, Z. Ullah, A. Sarwono, F.U. Shah, N. Muhammad, Efficient conversion of lignocellulosic biomass to levulinic acid using acidic ionic liquids, Carbohydr. Polym. 181 (2018) 208-214. [172] H. Kobayashi, Y. Ito, T. Komanoya, Y. Hosaka, P.L. Dhepe, K. Kasai, K. Hara, A. Fukuoka, Synthesis of sugar alcohols by hydrolytic hydrogenation of cellulose over supported metal catalysts, Green Chem. 13(2) (2011) 326-333. [173] R. Palkovits, K. Tajvidi, J. Procelewska, R. Rinaldi, A. Ruppert, Hydrogenolysis of cellulose combining mineral acids and hydrogenation catalysts, Green Chem. 12(6) (2010) 972-978. [174] A. Yamaguchi, O. Sato, N. Mimura, Y. Hirosaki, H. Kobayashi, A. Fukuoka, M. Shirai, Direct production of sugar alcohols from wood chips using supported platinum catalysts in water, Catal. Commun. 54 (2014) 22-26. [175] A. Yamaguchi, O. Sato, N. Mimura, M. Shirai, Catalytic production of sugar alcohols from lignocellulosic biomass, Catal. Today 265 (2016) 199-202. [176] C.V. Nguyen, D. Lewis, W.-H. Chen, H.-W. Huang, Z.A. Alothman, Y. Yamauchi, K.C.W. Wu, Combined treatments for producing 5-hydroxymethylfurfural (HMF) from lignocellulosic biomass, Catal. Today 278 (2016) 344-349. [177] L. Shuai, M.T. Amiri, Y.M. Questell-Santiago, F. Héroguel, Y. Li, H. Kim, R. Meilan, C. Chapple, J. Ralph, J.S. Luterbacher, Formaldehyde stabilization facilitates lignin monomer production during biomass depolymerization, Science 354(6310) (2016) 329-333. [178] W. Wang, M. Wang, J. Huang, X. Zhao, Y. Su, Y. Wang, X. Li, Formate-assisted analytical pyrolysis of kraft lignin to phenols, Bioresour. Technol. (2019). [179] I.S. Goldstein, Reactivity toward phenol of lignin from the hydrolysis of sweetgum wood with concentrated sulfuric acid, J. Wood Chem. Technol. 10(3) (1990) 379-386. [180] H. Kawamoto, Lignin pyrolysis reactions, J. Wood Sci. 63(2) (2017) 117-132. [181] K. Tang, X.-F. Zhou, The degradation of kraft lignin during hydrothermal treatment for phenolics, Pol. J. Chem. Technol. 17(3) (2015) 24. [182] J. Luo, P. Melissa, W. Zhao, Z. Wang, Y. Zhu, Selective lignin oxidation towards vanillin in phenol media, ChemistrySelect 1(15) (2016) 4596-4601. [183] A.W. Pacek, P. Ding, M. Garrett, G. Sheldrake, A.W. Nienow, Catalytic conversion of sodium lignosulfonate to vanillin: Engineering aspects. Part 1. Effects of processing conditions on vanillin yield and selectivity, Ind. Eng. Chem. Res. 52(25) (2013) 8361-8372. [184] M. Fache, B. Boutevin, S. Caillol, Epoxy thermosets from model mixtures of the lignin-to-vanillin process, Green Chem. 18(3) (2016) 712-725. [185] G. Wu, M. Heitz, E. Chornet, The depolymerization of lignin via aqueous alkaline oxidation, in: A.V. Bridgwater (Ed.), Advances in Thermochemical Biomass Conversion, Springer Netherlands, Dordrecht, 1993, pp. 1558-1571. [186] J. Zeng, C.G. Yoo, F. Wang, X. Pan, W. Vermerris, Z. Tong, Biomimetic Fentoncatalyzed lignin depolymerization to high-value aromatics and dicarboxylic acids, ChemSusChem 8(5) (2015) 861-71. [187] P. Azadi, O.R. Inderwildi, R. Farnood, D.A. King, Liquid fuels, hydrogen and chemicals from lignin: A critical review, Renewable Sustainable Energy Rev. 21 (2013) 506523.
64
Jo
ur
na
lP
re
-p
ro of
[188] M. Nagy, K. David, G.J.P. Britovsek, A.J. Ragauskas, Catalytic hydrogenolysis of ethanol organosolv lignin Holzforschung 63(5) (2009) 513-520. [189] J. Kang, S. Irmak, M. Wilkins, Conversion of lignin into renewable carboxylic acid compounds by advanced oxidation processes, Renewable Energy 135 (2019) 951-962. [190] N. Galeotti, F. Jirasek, J. Burger, H. Hasse, Recovery of Furfural and Acetic Acid from Wood Hydrolysates in Biotechnological Downstream Processing, Chemical Engineering & Technology 41(12) (2018) 2331-2336. [191] A. Martinez, M.E. Rodriguez, M.L. Wells, S.W. York, J.F. Preston, L.O. Ingram, Detoxification of dilute acid hydrolysates of lignocellulose with lime, Biotechnol Prog 17(2) (2001) 287-93. [192] G.-T. Jeong, S.-K. Kim, D.-H. Park, Detoxification of hydrolysate by reactiveextraction for generating biofuels, Biotechnology and Bioprocess Engineering 18(1) (2013) 88-93. [193] D.H. Cho, Y.J. Lee, Y. Um, B.I. Sang, Y.H. Kim, Detoxification of model phenolic compounds in lignocellulosic hydrolysates with peroxidase for butanol production from Clostridium beijerinckii, Applied microbiology and biotechnology 83(6) (2009) 1035-43. [194] T. Qu, X. Zhang, X. Gu, L. Han, G. Ji, X. Chen, W. Xiao, Ball milling for biomass fractionation and pretreatment with aqueous hydroxide solutions, ACS Sustainable Chem. Eng. 5(9) (2017) 7733-7742. [195] X. Yuan, S. Liu, G. Feng, Y. Liu, Y. Li, H. Lu, B. Liang, Effects of ball milling on structural changes and hydrolysis of lignocellulosic biomass in liquid hot-water compressed carbon dioxide, Korean J. Chem. Eng. 33(7) (2016) 2134-2141. [196] M.R. Zakaria, S. Fujimoto, S. Hirata, M.A. Hassan, Ball milling pretreatment of oil palm biomass for enhancing enzymatic hydrolysis, Appl. Biochem. Biotechnol. 173(7) (2014) 1778-89. [197] F. Shi, H. Xiang, Y. Li, Combined pretreatment using ozonolysis and ball milling to improve enzymatic saccharification of corn straw, Bioresour. Technol. 179 (2015) 444-451. [198] R.C.d.A. Castro, S.I. Mussatto, I.C. Roberto, A vertical ball mill as a new reactor design for biomass hydrolysis and fermentation process, Renewable Energy 114 (2017) 775780. [199] J.H. Lee, J.H. Kwon, T.H. Kim, W.I. Choi, Impact of planetary ball mills on corn stover characteristics and enzymatic digestibility depending on grinding ball properties, Bioresour. Technol. 241 (2017) 1094-1100. [200] B.-J. Gu, M.P. Wolcott, G.M. Ganjyal, Pretreatment with lower feed moisture and lower extrusion temperatures aids in the increase in the fermentable sugar yields from finemilled Douglas-fir, Bioresour. Technol. 269 (2018) 262-268. [201] H. Liu, B. Pang, Y. Zhao, J. Lu, Y. Han, H. Wang, Comparative study of two different alkali-mechanical pretreatments of corn stover for bioethanol production, Fuel 221 (2018) 2127. [202] B.-J. Gu, G.S. Dhumal, M.P. Wolcott, G.M. Ganjyal, Disruption of lignocellulosic biomass along the length of the screws with different screw elements in a twin-screw extruder, Bioresour. Technol. 275 (2019) 266-271. [203] C. Montiel, O. Hernández-Meléndez, E. Vivaldo-Lima, M. Hernández-Luna, E. Bárzana, Enhanced bioethanol production from blue agave bagasse in a combined extrusion– saccharification process, Bioenergy Res. 9(4) (2016) 1005-1014. [204] W.I. Choi, H.J. Ryu, S.J. Kim, K.K. Oh, Thermo-mechanical fractionation of yellow poplar sawdust with a low reaction severity using continuous twin screw-driven reactor for high hemicellulosic sugar recovery, Bioresour. Technol. 241 (2017) 63-69.
65
Jo
ur
na
lP
re
-p
ro of
[205] Y. Liu, L. Guo, L. Wang, W. Zhan, H. Zhou, Irradiation pretreatment facilitates the achievement of high total sugars concentration from lignocellulose biomass, Bioresour. Technol. 232 (2017) 270-277. [206] M. Molaverdi, K. Karimi, S. Mirmohamadsadeghi, Improvement of dry simultaneous saccharification and fermentation of rice straw to high concentration ethanol by sodium carbonate pretreatment, Energy 167 (2019) 654-660. [207] A. Sawisit, S. Jampatesh, S.S. Jantama, K. Jantama, Optimization of sodium hydroxide pretreatment and enzyme loading for efficient hydrolysis of rice straw to improve succinate production by metabolically engineered Escherichia coli KJ122 under simultaneous saccharification and fermentation, Bioresour. Technol. 260 (2018) 348-356. [208] X. Qi, L. Yan, F. Shen, M. Qiu, Mechanochemical-assisted hydrolysis of pretreated rice straw into glucose and xylose in water by weakly acidic solid catalyst, Bioresour. Technol. 273 (2019) 687-691. [209] K.-L. Chang, X.-M. Chen, Y.-J. Han, X.-Q. Wang, L. Potprommanee, X.-a. Ning, J.-y. Liu, J. Sun, Y.-P. Peng, S.-y. Sun, Y.-C. Lin, Synergistic effects of surfactant-assisted ionic liquid pretreatment rice straw, Bioresour. Technol. 214 (2016) 371-375. [210] H. Amiri, K. Karimi, H. Zilouei, Organosolv pretreatment of rice straw for efficient acetone, butanol, and ethanol production, Bioresour. Technol. 152 (2014) 450-456. [211] X. Xie, X. Feng, S. Chi, Y. Zhang, G. Yu, C. Liu, Z. Li, B. Li, H. Peng, A sustainable and effective potassium hydroxide pretreatment of wheat straw for the production of fermentable sugars, Bioresour. Technol. Rep. 3 (2018) 169-176. [212] Z. Yuan, G. Li, E.L. Hegg, Enhancement of sugar recovery and ethanol production from wheat straw through alkaline pre-extraction followed by steam pretreatment, Bioresour. Technol. 266 (2018) 194-202. [213] Z. Zhao, X. Chen, M.F. Ali, A.A. Abdeltawab, S.M. Yakout, G. Yu, Pretreatment of wheat straw using basic ethanolamine-based deep eutectic solvents for improving enzymatic hydrolysis, Bioresour. Technol. 263 (2018) 325-333. [214] X. Chen, H. Li, S. Sun, X. Cao, R. Sun, Co-production of oligosaccharides and fermentable sugar from wheat straw by hydrothermal pretreatment combined with alkaline ethanol extraction, Ind. Crops Prod. 111 (2018) 78-85. [215] H. Zhang, G. Lyu, A. Zhang, X. Li, J. Xie, Effects of ferric chloride pretreatment and surfactants on the sugar production from sugarcane bagasse, Bioresour. Technol. 265 (2018) 93-101. [216] Z. Zhu, C.A. Rezende, R. Simister, S.J. McQueen-Mason, D.J. Macquarrie, I. Polikarpov, L.D. Gomez, Efficient sugar production from sugarcane bagasse by microwave assisted acid and alkali pretreatment, Biomass Bioenergy 93 (2016) 269-278. [217] P.F. Avila, M.B.S. Forte, R. Goldbeck, Evaluation of the chemical composition of a mixture of sugarcane bagasse and straw after different pretreatments and their effects on commercial enzyme combinations for the production of fermentable sugars, Biomass Bioenergy 116 (2018) 180-188. [218] M. Espirito Santo, C.A. Rezende, O.D. Bernardinelli, N. Pereira, A.A.S. Curvelo, E.R. deAzevedo, F.E.G. Guimarães, I. Polikarpov, Structural and compositional changes in sugarcane bagasse subjected to hydrothermal and organosolv pretreatments and their impacts on enzymatic hydrolysis, Ind. Crops Prod. 113 (2018) 64-74. [219] H. Yu, Y. You, F. Lei, Z. Liu, W. Zhang, J. Jiang, Comparative study of alkaline hydrogen peroxide and organosolv pretreatments of sugarcane bagasse to improve the overall sugar yield, Bioresour. Technol. 187 (2015) 161-166. [220] J. Li, W. Li, M. Zhang, D. Wang, Boosting the fermentable sugar yield and concentration of corn stover by magnesium oxide pretreatment for ethanol production, Bioresour. Technol. 269 (2018) 400-407. 66
Jo
ur
na
lP
re
-p
ro of
[221] L. Qin, X. Li, J.-Q. Zhu, W.-C. Li, H. Xu, Q.-M. Guan, M.-T. Zhang, B.-Z. Li, Y.-J. Yuan, Optimization of ethylenediamine pretreatment and enzymatic hydrolysis to produce fermentable sugars from corn stover, Ind. Crops Prod. 102 (2017) 51-57. [222] Q. Qing, L. Zhou, Q. Guo, X. Gao, Y. Zhang, Y. He, Y. Zhang, Mild alkaline presoaking and organosolv pretreatment of corn stover and their impacts on corn stover composition, structure, and digestibility, Bioresour. Technol. 233 (2017) 284-290. [223] H. Teramura, K. Sasaki, T. Oshima, H. Kawaguchi, C. Ogino, T. Sazuka, A. Kondo, Effective usage of sorghum bagasse: Optimization of organosolv pretreatment using 25% 1butanol and subsequent nanofiltration membrane separation, Bioresour. Technol. 252 (2018) 157-164. [224] Y. Jafari, H. Amiri, K. Karimi, Acetone pretreatment for improvement of acetone, butanol, and ethanol production from sweet sorghum bagasse, Appl. Energy 168 (2016) 216225. [225] R. Choudhary, A.L. Umagiliyage, Y. Liang, T. Siddaramu, J. Haddock, G. Markevicius, Microwave pretreatment for enzymatic saccharification of sweet sorghum bagasse, Biomass Bioenergy 39 (2012) 218-226. [226] J. Hu, V. Arantes, J.N. Saddler, The enhancement of enzymatic hydrolysis of lignocellulosic substrates by the addition of accessory enzymes such as xylanase: is it an additive or synergistic effect?, Biotechnol. Biofuels 4(1) (2011) 36. [227] H. Inoue, S.R. Decker, L.E. Taylor, S. Yano, S. Sawayama, Identification and characterization of core cellulolytic enzymes from Talaromyces cellulolyticus (formerly Acremonium cellulolyticus) critical for hydrolysis of lignocellulosic biomass, Biotechnol. Biofuels 7(1) (2014) 151. [228] Y. Yang, J. Yang, J. Liu, R. Wang, L. Liu, F. Wang, H. Yuan, The composition of accessory enzymes of Penicillium chrysogenum P33 revealed by secretome and synergistic effects with commercial cellulase on lignocellulose hydrolysis, Bioresour. Technol. 257 (2018) 54-61. [229] R. Kumar, C.E. Wyman, Effects of cellulase and xylanase enzymes on the deconstruction of solids from pretreatment of poplar by leading technologies, Biotechnol. Progr. 25(2) (2009) 302-314. [230] K. Murashima, A. Kosugi, R.H. Doi, Synergistic effects of cellulosomal xylanase and cellulases from Clostridium cellulovorans on plant cell wall degradation, J. Bacteriol. 185(5) (2003) 1518-1524. [231] A. Duque, P. Manzanares, M. Ballesteros, Extrusion as a pretreatment for lignocellulosic biomass: Fundamentals and applications, Renewable Energy 114 (2017) 1427-1441. [232] R. Ma, M. Guo, X. Zhang, Selective conversion of biorefinery lignin into dicarboxylic acids, ChemSusChem 7(2) (2014) 412-415.
67
Table 1 Major ongoing R&D projects on biofuels in India.
2.
R&D Institute Enhanced butanol production NEERI, from lignocellulosic biomass Nagpur using improved pretreatment and integrated saccharification, fermentation and separation in a membrane bioreactor Development of pretreatment DU, New strategies and bioprocess for Delhi improved production of cellulolytic enzymes and ethanol from crop byproduct for demonstration at pilot plant
Sanctioned Year 2011
Sanctioned amount INR (USD) 39.7 lakhs (55,810)
2012
148.5 lakhs (2,08,761)
ro of
1.
Project Title
-p
Sl. No.
Process development for bioethanol production from agricultural residues PhaseI: Development of process for co-fermentation of hexose and pentose sugars of agricultural residues
SSSNRE, Punjab
4.
Hydropyrolysis of lignocellulosic biomass to value-added hydrocarbons
IIP, Dehradun
2012
186.4 lakhs (2,62,144)
5.
Sorghum stover based biorefinery for fuels and chemicals
NIIST, Trivandrum; MNNIT, Allahabad; TERI, New Delhi & IICT, Hyderabad JCE, Mysore
2012
184.14 lakhs (2,58,965)
2012
24 lakhs (33,742)
lP
na
ur
Jo 6.
Isolation and characterization of Diatom species and process development for biofuel production
2012
132.2 lakhs (1,85,919)
re
3.
68
Engineering Escherichia coli strains optimized for large scale lignocelluloses fermentation for biofuel production
IIT, Bombay
2012
25 lakhs (35,148)
8.
Stabilization and up gradation of biomass derived bio-oils over tailored multifunctional catalysts in a dual stage catalytic process to produce liquid hydrocarbon fuels and its application studies
TERI, New Delhi
2013
164 lakhs (2,30,568)
9.
Improved production of biogas and bio-CNG from lignocellulosic biomass
10.
Development of new catalytic systems for efficient transformation of biomass/biomass derivatives to biofuel components
DBT-ICT 2013 Centre for Energy Biosciences, ICT, Matunga (E), Mumbai; Kirloskar Integrated Technologies Limited, Pune; Glycols Limited, Noida IIT, Indore 2017
ro of
7.
Jo
ur
na
lP
re
-p
445.90 lakhs (6,26,779)
69
54.15 lakhs (76,119)
Table 2 Effect of ball milling on pretreatment of biomass. Sl. No.
Biomass
Pretreatment method
Major effects
Ref
1.
Wheat straw
Ball milling with alkali hydrolysis
[194]
2.
Camphorwood Ball milling with saw dust carbon-di-oxide catalyzed hydrothermal treatment
3.
Oil palm fruit fiber
Ball milling
4.
Corn straw
Ball milling
-Ultrafine structure of biomass was formed -Removal of hemicellulose and lignin was 93.8% and 86.1% respectively -A maximum glucose of 98.5% was obtained after 72 h of enzymatic hydrolysis -Cellulose crystallinity was decreased to 21.4% from 60.9% of raw biomass -Conversion of cellulose was 37.8% -A maximum glucose of 1.5 g/L was obtained in the hydrolysate -After 60 min of milling at 250 rpm, 80.3% glucose and 78.6% xylose was obtained from enzymatic hydrolysis - After 60 min of milling at 250 rpm, 89.6% reduction in size of the biomass attributes to 4.5% of crystallinity index (CI) compared to 57% of CI for untreated biomass -86.4% reduction in average particle size of the biomass when it was milled at 1332 rpm for 12 min while specific surface area was increased by 1143% compared to 26.68 m2/kg for untreated biomass -48% reduction in crystallinity was achieved under the above condition -26.5% of glucose and 10% of xylose was produced after saccharification of biomass milled for 8 min -A maximum cellulose conversion of 87% was observed from 24 h of enzymatic hydrolysis with the biomass milled at 100 rpm using 30 unit of glass sphere in the reaction system
ro of
[195]
Jo
ur
na
lP
re
-p
[196]
5.
Rice straw
Ball milling using vertical reactor
70
[197]
[198]
Corn stover
[199]
Jo
ur
na
lP
re
-p
ro of
6.
-A maximum of 21.7 g/L ethanol was produced with 100% of glucose uptake during fermentation of ball mill treated biomass Planetary ball milling -Steel ball produced 73% of using steel, zicornia particle (<100 µm) when biomass and alumina ball was milled at 300 rpm for 20 min while alumina and zicornia produced only 52% of biomass particle -Compared to steel and zicornia, biomass milled for 60 min with alumina ball leads to produce a maximum of 92% of glucose after enzymatic hydrolysis
71
Table 3 Effect of extrusion pretreatment on lignocellulosic biomass. Variables -Temperature (25, 50, 100, 150 °C) -Moisture (30, 40, 50 % (w/w))
2.
Corn stover
-Reaction period of enzymatic hydrolysis
3.
Doulas fir
4.
Agave bagasse
-Maximum glucose of 39.3% [202] was obtained after enzymatic hydrolysis of extruded biomass -Lowest screw speed of 25 rpm produced maximum sugar yield of ~37% -Reverse screw element significantly improved sugar yield after enzymatic hydrolysis of biomass -Enzyme - A maximum of ~85% [203] dosage at hydrolysis yield was obtained different when 5% (w/v) extruded consortium biomass was treated with -Variation in Celic CT2 enzyme for72 h the -In the presence of Celic CT2 concentration enzyme, 20% (w/w) biomass of pretreated showed 73% of hydrolysis biomass yield with a production of around 70 g/L glucose
re
-Temperature of barrel (50, 100, 150 °C) -Screw speed (25, 50, 75 rpm)
na
lP
Extrusion followed by chemical pretreatment with NaOH at 0.06 g NaOH/g biomass at liquid to solid loading of 2:1 Extrusion using twin screw extruder with length to diameter of 40.5 for biomass contained 50% of moisture content
Jo
ur
Extrusion using twin screw extruder constructed with 10 different modules specific for the subsequent functions of feeding, NaOH treatment, neutralization, filtration and compression
Major effect of pretreatment Ref -Maximum sugars of ~42% [200] was obtained at lowest moisture content of 30% and lowest temperature of 25 °C -Compared to the other decrement, a significant decrement in the crystallinity was observed at 25 °C temperature and biomass of 40% moisture content -Delignification of biomass [201] was 69.5% -Total sugar yield was 78.8% after 24 h of enzymatic hydrolysis
ro of
Biomass Pretreatment Douglas Extrusion using fir twin screw extruder with 24 screw elements and 1 reverse element at speed of 25 rpm
-p
Sl. No. 1.
72
-Hemicellulosic sugars (xylose, mannose and galactose) showed a maximum of 74.8% yield at 127 °C and with an addition of 0.8% (w/w) acid concentration -Double fold increase in surface was observed for extruded sample compared to untreated biomass (0.34 m2/g)
[204]
ro of
-Temperature of barrel (115 -160 °C) -Acid concentration (0.5 – 1.4% (w/w))
Jo
ur
na
lP
re
-p
5.
using two reverse screws rotated at 100 rpm Yellow Continuous poplar twin screw saw dust driven reactor was constituted of barrel. Shaft, screw, feeding system and drive motor with length/diameter of the screw in the reaction zone was 34.4 and rotation was fixed at 25 rpm
73
Table 4 Assessment of the effect of irradiation pretreatment on the various parameters for different biomass.
Sugarcane bagasse
Pretreatment method Gamma radiation
Electron beam radiation with acid hydrolysis
Lignin content
Kenaf core
Electron beam radiation with acid hydrolysis
Crystallinity index
Sugarcane bagasse
Effect of pretreatment Compared to 45.4% (w/w) in the raw biomass, cellulose content decreased to 11.2% (w/w) at 2000 kGy of gamma radiation 500 kGy of Compared to 25.4% electron beam (w/w) present in dose with 3% raw biomass, (v/v) sulphuric hemicellulose acid content of the pretreated biomass was observed as 1.4% (w/w) -50 kGy of Sole pretreatment electron beam with electron beam -Acid hydrolysis or acid dosing with 3% (v/v) didn’t produce any sulphuric acid significant changes -500 kGy of in lignin content of electron beam pretreated biomass. dose with 3% However, at high (v/v) sulphuric dosing of electron acid beam with sequential acid hydrolysis resulted in 45.1% reduction in lignin content More than 100 Compared to raw kGy of gamma biomass 30% radiation reduction in crystallinity index was observed at 500 kGy of gamma radiation 800 kGy of Following gamma radiation irradiation, and subsequent consumption of milling energy during milling was around 120 kWh/ton of oven dried biomass
Ref [59]
[57]
[57]
Jo
ur
na
lP
re
Hemicellulose Kenaf core content
Reaction condition More than 100 kGy of gamma radiation
ro of
Biomass
-p
Parameter studied Cellulose content
Milling energy consumption
Gamma radiation
Cedarwood Gamma radiation
74
[59]
[205]
while for unirradiated biomass it was more than 400 kWh/ton of oven dried biomass
Pine
Gamma radiation
800 kGy of gamma radiation and subsequent milling
After irradiation, following milling for 1 min, less than 180 µm particle size of around 55.9% (w/w) was obtained compared to 35.5% (w/w) of untreated biomass milled for 4 min
Jo
ur
na
Eucalyptus
lP
re
-p
Particle size
Following irradiation, consumption of energy during milling was around 100 kWh/ton of oven dried biomass while for unirradiated biomass it was more than 400 kWh/ton of oven dried biomass
ro of
Pine
75
After irradiation, following milling for 1 min, less than 180 µm particle size of around 46.7% (w/w) was obtained compared to 37.9% (w/w) of untreated biomass milled for 4 min
[205]
Table 5 Effect of different physicochemical pretreatment methods on the saccharification of lignocellulosic residues. Remarks on Saccharification -Conversion of 91% and highest glucose yield of 124.7 g/L
Ref
1.
Rice straw
-0.5 M Na2CO3 at 93 °C for 5 h -2.0 M NaOH at 121 °C for 15 min
-After 5 days of enzymatic hydrolysis, 63.4% (w/w) total sugars was obtained
[207]
-6% (w/w) KOH treated 0.1 g biomass with 0.05 g C (generated from pretreated hydrolysate) ball milled for 2 h and subsequent hydrolysis with water at 200 °C for 60 min
-19.4% (w/w) glucose [208] and 61.5% (w/w) xylose was obtained
-After 72 h of enzymatic hydrolysis, 5.2 g/L of reducing sugar was obtained with cellulose conversion of ~35%
-75% (v/v) aqueous ethanol with 1% (w/w) sulphuric acid at 180 °C for 30 min -15% KOH, 120 °C for 40 min
-15.1 g/L glucose, 5.6 [210] g/L xylose and 1 g/L arabinose was obtained after 72 h of enzymatic hydrolysis -80% yield in total [211] sugars with ~90 % and ~70% yield of glucose and xylose respectively at 12 h of enzymatic hydrolysis
-Sequential treatment with 8% (w/w) NaOH at 80° C for 90 min followed by steam explosion with 3% (w/w) SO2 at 151 °C for
-80% (w/w) glucose and 65% (w/w) xylose was obtained at 25% (w/v) of solid loading during enzymatic hydrolysis
ur
na
lP
-Biomass was pretreated with 5 g ([BMIM]Cl) as ionic liquid at 110 °C for 60 min in the presence of SDS as a surfactant
Wheat straw
Jo
2.
[206]
ro of
Pretreatment
-p
Biomass
re
Sl. No.
76
[209]
[212]
16 min [213]
-Hydrothermal pretreatment with water at 180 °C for 30 min
-Pretreated filtrate was obtained with 6 g and 60 g of xylose and xylooligosaccharides respectively per kg of biomass and enzymatic hydrolysis of pretreated biomass reported 55% of glucan conversion 80.1% of glucose yield was observed after 72 h of enzymatic hydrolysis
[214]
-86% sugar yield was observed in which 64% was glucose
[216]
-[EMIM]Ac as an ionic -99.5% and 86.5% of liquid at 25 °C for 30 glucose and xylose min in an ultrasonic bath was reported after enzymatic hydrolysis
[217]
-Hot water pretreatment with 0.025 M FeCl3 at 160 °C for 10 min
[215]
na
lP
re
-Microwave assisted heating for 7 min in the presence of 0.2 M H2SO4
ro of
Sugarcane bagasse
-90% of cellulose and 62% of xylan conversion in enzymatic hydrolysis
-p
3.
-Choline chloride: monoethanolamine as deep eutectic solvent, 70 °C for 9 h
-76.8% enzymatic hydrolysis yield was obtained based on cellulose conversion into glucose
-Green liquid in combinations with ethanol-water [50:50% (v/v)] at 140 °C for 4 h
-Glucose and xylose [219] yield of 97.7% and 94.1% respectively was obtained after 72 h of enzymatic hydrolysis -71.5% (w/w) glucose [65] yield was reported after 72 h of enzymatic hydrolysis
Jo
ur
-Water as hydrothermal media at 160 °C for 30 min followed by 50% (v/v) ethanol-water as organosolv media at 190 °C for 100 min
4.
Corn stover
-Dilute acid in 1% (w/w) HCl at 140 °C for 40 min in combination with ammonia wet 77
[218]
oxidation under the condition 12% (w/w) NH4OH, 3 MPa O2 at 130 °C 40 min
-2 g/g solid NH3 loading at 60 °C for 2 d in coupled with ethylenediamine loading of 0.6 mL/ g of biomass at 100 °C for 20 min
-28.8 g/L glucose and 11 g/L of xylose was obtained after 72 h of enzymatic hydrolysis
[221]
-Presoaked in 1% (w/w) Na2S at 40 °C for 4 h followed by organosolv pretreatment with 0.2% (w/v) HCl in 20% methanol at 160 °C for 20 min -25% (v/v) 1-butanol in 0.5% (w/w) H2SO4 at 200 °C for 60 min
-More than 80% yield in total sugars were reported after 72 h of enzymatic hydrolysis.
-p
ro of
[220]
[222]
-About 95% of glucose yield was obtained after 72 h of enzymatic hydrolysis
[223]
-50% (v/v) acetonewater with 0.1% (w/w) H2SO4 at 180 °C for 60 min
-78 g/L glucose and 12 g/L xylose was obtained after 72 h of enzymatic hydrolysis
[224]
-Microwave treatment with 10 mL/ g of biomass for 4 min
-40% (w/w) total reducing sugars was reported after 72 h of enzymatic hydrolysis
[225]
re
Sorghum bagasse
-Total sugar of 50 g/L was obtained after 120 h of enzymatic hydrolysis
Jo
ur
na
lP
5.
-0.1 mol/L MgO at 190 °C for 40 min
78
Table 6 Effect of mixed consortium of enzymes on the saccharification of biomass. Enzyme cocktail
Lignocellulose
Effect on hydrolysis 87%a
Ref
1.
Cellulase (Celluclast 1.5 L) + multifect xylanase
Steam pretreated corn stover (SPCS)
2.
Multifect xylanase + cellulase (Celluclast 1.5 L)
SPCS
100% a1
[226]
3.
Core enzyme mixture (Cel7A, Cel6A, Cel5A, Xyl10A, Bgl3A)
Pretreated corn stover
80% b
[227]
4.
Cellulase (T. longibrachiatum) + P33 enzyme cocktail (P. chrysogenum)
Corn stover
5.
Spezyme CP cellulase + Multifect xylanase
Dilute acid pretreated poplar solid
58.2%e,f
[229]
6.
Xylanase XynA + cellulase
Corn cell wall
0.622 µmol/ming
[230]
lP
re
-p
83%c , 44%d
[228]
Cellulose of SPCS converted after 72 h of enzymatic hydrolysis; a1 Xylan of SPCS converted after 72h of enzymatic hydrolysis; b After 48 h of enzymatic hydrolysis; c Xylan conversion after 120 h of enzymatic hydrolysis; d Glucan conversion after 120 h of enzymatic hydrolysis; e After 24 h of hydrolysis; f Glucose released; g After 15 h of enzymatic reaction
Jo
ur
na
a
[226]
ro of
Sl. No.
79
Table 7 List of companies involved in the production of biofuel from biomass. Country
Biomass used
1.
Poet Dsm (http://www.poetdsm.com, 12 March 2019)
USA
Corn crop
2.
Abengoa bioenergy (http://www.abengoa.com, 12 March 2019)
Spain & USA
3.
Clariant (http://www.clariant.com, 12 March 2019)
Switzerland
wheat, barley, corn, sorghum, corn stover, wheat straw, oat straw, barley straw, hardwood, switchgrass wheat straw, rice straw, corn stover and sugar cane bagasse
4.
Raizen-Iogen Brazil (https://www.raizen.com.br) (http://www.iogen.ca, 12 March 2019)
5.
Ethtec Australia (https://www.ethtec.com.au, 12 March 2019)
-p re
lP na
ur
Jo
80
Recent breakthrough Novel approach in the pretreatment of corn feedstock excels the production of cellulosic ethanol Innovative enzymatic process manifests 25 million gallon of ethanol per year
ro of
Sl. No. Name of the company
Sugarcane bagasse and straw
Crop stubbles, cotton gin trash, timber residues, sugarcane bagasse
Recent partnership with Exxonmobil and renewable energy group jointly venture on integrated pretreatment process towards cost effective biodiesel production The plant located at Piracicaba successfully installed 42 million liters of production unit of bioethanol Fund disbursement of $11.9 million dollar by Australian renewable energy agency to Ethtec enable the company to build the latest pilot lignocellulosic
fuel plant in the Hunter valley region Gevo USA (https://www.gevo.com) (http://www.renmatix.com, 12 March 2019)
Wood, agricultural residues
Renmatix plantrose process in a joint venture with Geno’s GIFT technology was aimed to evaluate the feasibility of producing jet fuel alcohol
Jo
ur
na
lP
re
-p
ro of
6.
81
Table 8 Comparison among various fermentation methods available for lignocellulosic biomass.
ur
Simultaneous pretreatment and saccharification
Jo
3.
4.
Consolidated bioprocessing
-Pretreatment and saccharification of biomass is carried out together followed by fermentation -Pretreatment, saccharification and fermentation process of 82
ro of
Demerits -Product inhibition due to prolonged hydrolysis -Cost intensive process - Overall time consuming process -Chances of contamination is high
-p
Simultaneous saccharification and fermentation
Merits -The optimum conditions for hydrolysis and fermentation is appropriately maintained as both of the reactions are carried out in separate reactors
re
2.
Features -After pretreatment, biomass is hydrolyzed at optimum reaction conditions favorable for enzyme -The hydrolysate obtained after enzymatic hydrolysis is utilized as feedstock for fermentation at reaction conditions favorable for fermentation -After an initial pretreatment process, biomass is hydrolyzed and fermented simultaneously
lP
Method Separate hydrolysis and fermentation
na
Sl. No. 1.
-Chances of product inhibition is less -Process cost is less as no separate reactor is required for subsequent hydrolysis and fermentation process -Less complex process -Process cost is low -Chances of sugars loss is low
-Difficult to maintain the favorable reactions condition as the temperature required for enzymatic hydrolysis is 50 °C while for fermentation 37 °C has been an optimum
-No complexity involved in the process -Economically most favorable
-Different optimum conditions for different reactions involved during the bioprocess.
-Difficult to maintain different optimum conditions both for pretreatment and saccharification
biomass is carried out in a single reactor with the presence of microbes.
Solid state fermentation
-Scale up of this process is limited at bench scale level -Problem with heat and mass transfer
Jo
ur
na
lP
re
-p
ro of
5.
as extraneous addition of enzyme is not required thus save the maximum cost of the overall process -The process -Higher yield facilitates the -Efficient growth of the utilization of microorganisms biomass as sole on solid carbon source materials at low -Improved moisture product content characteristics
83
ro of -p re
Jo
ur
na
lP
Fig. 1. Pictorial representation of the overall process involved in the production in biofuel from biomass.
84
Amount sanctioned in INR
300 2015-16
250
2016-17
2017-18
200 150 100 50 0 MOP
DST
CSIR
MNRE
MOPNG
DBT
ro of
Government agencies
Jo
ur
na
lP
re
-p
Fig. 2. Budgetary allocations implemented through different R&D agencies of Government of India in the recent years. [MOP: Ministry of Power; DST: Department of Science & Technology; CSIR: Council of Scientific & Industrial Research; MNRE: Ministry of New & Renewable Energy; MOPNG: Ministry of Petroleum and Natural Gas; DBT: Department of Biotechnology] (Source: Mission Innovation: India-Country Report and Progress Update, May 2018).
85
ro of
Jo
ur
na
lP
re
-p
Fig. 3. Design of a typical twin screw extruder. A) Motor; B) Hopper; C) Thermal regulated barrels; D) Screws; E) Pumps. Reprinted with permission from [231].
86
ro of -p re lP
Jo
ur
na
Fig. 4. Schematic representation of enzymatic hydrolysis of biomass.
87
Jo
ur
na
lP
re
-p
ro of
Fig. 5. Proposed mechanism of acetic acid formation from carbohydrate [167].
88
ro of -p re lP
Jo
ur
na
Fig. 6. Lignin depolymerization and aromatic nuclei oxidation (A); Aromatic ring cleavage (B); Formation of carboxylic acids (C). Reprinted with permission from John Wiley and Sons [232].
89
PII:
S1359-5113(19)31090-6
DOI:
https://doi.org/10.1016/j.procbio.2019.10.001
Reference:
PRBI 11789
To appear in:
Process Biochemistry
Received Date:
17 July 2019
Revised Date:
14 September 2019
Accepted Date:
3 October 2019
Please cite this article as: Haldar D, Purkait MK, Lignocellulosic conversion into value-added products: A review, Process Biochemistry (2019), doi: https://doi.org/10.1016/j.procbio.2019.10.001
This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier.
Lignocellulosic conversion into value-added products: A review Dibyajyoti Haldar*, Mihir Kumar Purkait* Centre for the Environment, Indian Institute of Technology Guwahati, Assam-781039, India
---------------------------------------*
ro of
Corresponding authors Dr. Mihir Kumar Purkait ([email protected]), Professor, Department of Chemical Engineering, IIT Guwahati, India. Dr. Dibyajyoti Haldar ([email protected]), IPDF, Centre for the Environment, IIT Guwahati, India.
Highlights
re
-p
Involvement of several global agencies to strengthen biofuel production. Insight into the progress in various pretreatment technologies. Aspect of cellulase for an effective saccharification process. Genetic and metabolic engineering towards an enhanced biofuel production. Fate of lignocellulosic conversion into the derivatives of cellulose and lignin.
Jo
ur
na
lP
1
Abstract In the present context of energy crisis, exploration of lignocellulosic biomass has emerged as potential substitute to maintain environmental sustainability. However, the conversion of biomass into value-added products still faces challenges to find a suitable unit operation. The stubborn dependency on the cost intensive enzymatic system, limits an effective saccharification to hydrolyze the biomass. India has been one of the top most agriculturally enriched countries with broad scope of utilizing waste residue that remains as an unused
ro of
biomass at harvested locations. Hence, in the present script, an overview on the latest R&D initiatives taken by Government of India are briefed to highlight the projects based on biofuels and in addition, global scenario of biofuel production is comprehensively discussed.
-p
Further, critical analysis on the advancement of different pretreatment operations are highlighted through latest inventions. Thereafter, biochemistry of cellulase enzyme with
re
essential factors were explored to understand the mechanistic interactions involved during
lP
saccharification of biomass. An insight on the latest accomplishments of various fermentation process provides an in depth understanding of metabolic engineering based on the genetic
na
studies of fermentative microorganisms. Finally, the article is concluded with brief discussions on fate of the derivatives obtained from macromolecules such as cellulose and
ur
lignin.
Key words: Lignocellulose; Biomass conversion; Value-added products; Hydrolysis;
Jo
Fermentation.
2
List of abbreviations R&D: Research and Development SHF: Separate Hydrolysis and Fermentation SSF: Simultaneous Saccharification and Fermentation ABE: Acetone Butanol Ethanol CBP: Consolidated Bioprocessing HMF: Hydroxymethyl Furfural
ro of
CO2: Carbon-Di-Oxide DBT: Department of Biotechnology NREL: National Renewable Energy Laboratory
-p
IOCL: Indian Oil Corporation Limited ICT: Institute of Chemical Technology
re
BPCL: Bharat Petroleum Corporation Limited
lP
MOP: Ministry of Power
DST: Department of Science and Technology
na
MNRE: Ministry of New and Renewable Energy
CSIR: Council of Scientific and Industrial Research
ur
MOPNG: Ministry of Petroleum and Natural Gas DP: Degree of Polymerization
Jo
CBD: Cellulose Binding Domain CD: Catalytic Domain CBH: Cellobiohydrolase NADH: Nicotinamide Adenine Dinucleotide Hydrogen
3
Contents 1.
Jo
ur
5.
na
lP
4.
re
3.
-p
ro of
2.
Introduction 1.1. Present scenario of biofuel production in India 1.2. Scenario on global production of biofuel Conventional pretreatment methods 2.1. Pretreatment with aid of physical methods 2.1.1. Pretreatment using ball milling 2.1.2. Pretreatment using extrusion 2.1.3. Pretreatment with the aid of irradiation 2.2. Pretreatment with aid of chemicals 2.2.1. Pretreatment of lignocellulosic biomass using acid 2.2.2. Pretreatment of lignocellulosic biomass using alkali 2.2.3. Pretreatment of lignocellulosic biomass using ionic liquid 2.2.4. Pretreatment of lignocellulosic biomass using organic solvents 2.3. Physico-chemical pretreatment 2.3.1. Steam explosion pretreatment 2.3.2. Liquid hot water pretreatment 2.3.3. Wet oxidation pretreatment 2.4. Biological pretreatment 2.5. An insight on latest and advanced emerging pretreatment technologies Strategies towards improved enzymatic hydrolysis of biomass 3.1. Factors influence the process of enzymatic hydrolysis 3.2. Cellulase enzyme: Biochemistry and mechanistic interactions 3.3. Efforts to overcome the criticalities involved with cellulase Strategies involved in the fermentation of sugars into biofuel 4.1. Consolidated bioprocess (CBP) involved in fermentation 4.2. Simultaneous saccharification and co fermentation 4.3. Simultaneous pretreatment and saccharification 4.4. Exploration of metabolic engineering towards improved biofuel production 4.4.1. Engineered saccharomyces cerevisiae for bioethanol production 4.4.2. Engineered Zymomonas mobilis for bioethanol production 4.4.3. Genetic engineering in Clostridium acetobutylicum for biobutanol production Conversion of biomass into the derivatives of cellulose and lignin 5.1. Derivatives of cellulose 5.1.1. Formic acid 5.1.2. Acetic acid 5.1.3. Levulinic acid 5.1.4. Sugar alcohol 5.1.5. Hydroxymethyl furfural (HMF) 5.2. Derivatives of lignin 5.2.1. Phenol 5.2.2. Vanillin 5.2.3. Carboxylic acids 5.3. Detoxification of inhibitors Future perspective Conclusions
6. 7.
4
1. Introduction In order to meet the growing demand of renewable energy, adequate sources of fossil fuels are inadequately utilized as the most vulnerable resource of this globe [1-3]. Besides, rapid urbanization including global industrialization is equally anticipated with fast exploitation of the fossil fuels. Therefore, alternative search towards the production of green fuels using renewable feedstock like lignocellulosic biomass has been a potential choice as the biomass does not necessarily compete with conventional food crops. Further, the process involved in
ro of
the production of biofuel (e.g., bioethanol, biobutanol) from biomass has been of low toxicity and cost effective. Hence, with regard to an industrial acceptability, advancements in the process of pretreatment, saccharification, and fermentation has been encouraging as top
-p
research endeavor.
Pretreatment has been an essential unit operation in order to beak the outermost seal of lignin
re
and subsequent exposure of carbohydrates (cellulose and hemicellulose) towards the process
lP
of hydrolysis [4-6]. Primarily, the process of pretreatment largely dictates the overall cost of the technology as it directly deals with heterogeneous sources of lignocellulosic biomass [7,
na
8]. Therefore, a number of different pretreatment methods have always emerged as motivation to architect an optimum reaction condition for each of the individual feedstock
ur
materials. With regard to the nature of the reaction, pretreatment can be subcategorized into different classes as physical, chemical, physico-chemical and biological processes [9-11].
Jo
Physical pretreatment attributes towards size reduction of biomass particle with an increase in an effective surface area manifesting a reduction in degree of polymerization and physicochemical pretreatment involves chemical addition along with combination of physical processes [12, 13]. Among different physico-chemical pretreatment methods, steam explosion, liquid hot water and wet oxidation are mostly studied [14]. Chemical pretreatment involves an incorporation of different chemicals such as acid, alkali, organic solvents and 5
liquid mixture of ions in the reaction mixture. Finally, biological pretreatment uses a number of several microorganisms that excrete enzymes to deconstruct the rigid structure of biomass [15]. Each of the pretreatment methods has their own limitations. An attempt towards the robust deconstruction of biomass materials through pretreatment drives towards advancement in the process based on the existing procedures. Following pretreatments, hydrolysis using enzyme assists in the production of monosugars from biomass [16]. A number of several attempts are made to ease out the high cost
ro of
associated with enzyme [17]. Though enzymatic hydrolysis is not resulted into the formation of inhibitors but the inhibitor free formation of sugars partially inhibits action of the enzyme due to product inhibition during the process of separate hydrolysis and fermentation (SHF).
-p
On the other side, simultaneous saccharification and fermentation is conducted using a single reactor to overcome the problem of product inhibition and improved the product yield within
re
a short span of time [18-20]. Fig. 1 depicts the overall process involved in the production of
lP
biofuel from biomass which represents the schematic of each of the individual steps of pretreatment, hydrolysis and fermentation process. Lignocellulosic biomass has been the reservoir of carbon in different polymeric forms of
na
cellulose, hemicellulose and lignin [21-23]. Therefore, the biomass is subjected to be utilized as a potent sustainable bioresource, once it is transformed economically into value-added
ur
products. The technologies involved in the bioconversion process primarily focuses on sole
Jo
production of the specific valuable product [24-26]. Formation of selective derivatives generated from cellulose and lignin still be a promising approach in order to get rid of additional cost during separation in downstream processing. Exhaustive explanations on various pretreatment techniques are readily available in the literatures to evaluate the performance of various uniform or combined chemical and physicochemical operations for disintegration of biomass. In view of that, Sarkar et al.
6
reviewed different pretreatment methods for the production of bioethanol from agricultural residues with mere highlights on each of the pretreatment methods [27]. Moreover, Chen et al. provides an exhaustive narration on various pretreatment methods for lignocellulosic conversion into high value chemicals without detailing the formation of any value-added chemicals as a result of pretreatment conversion [28]. Further, Alvira et al. highlights on the key factors influence the process of the pretreatment technologies for an efficient conversion into bioethanol, based on enzymatic hydrolysis [29]. However, none of the articles on
ro of
pretreatment process provides any in-depth discussion on any of the particular unit operation. Whereas, in our present article at the very beginning of pretreatment section an emphasize was given on various aspects of mechanical pretreatments with an aspiration of exploring the
-p
promising ability of ball milling and extrusion towards an effective deconstruction of biomass. Additionally, recent advancements in each of the specific unit operations were
re
explored to determine the progress specifically in the sector of pretreatment. Further, a
lP
number of different approaches were attempted to hydrolyze the biomass using a variety of cellulolytic enzymes. In view of that, a clear mechanistic insight on the biochemistry of cellulase enzyme will facilitate to select the type of the target specific enzyme required for a
na
particular biomass. Following hydrolysis, the processed biomass is subjected to fermentation process for value-addition. Further, instead of mostly reported conventional approaches of
ur
SHF and SSF, combinations of different fermentations techniques with a better genetic and
Jo
metabolic understanding of microorganism seems beneficial to intensify the process with respect to biofuel production from biomass. In view of an improved production, various routes of fermentation demands an in-depth assessment on several factors. Yang et al. observed various challenge and prospects involved with solid-state anaerobic digestion for an efficient conversion of lignocellulosic biomass [30]. However, the promising aspect of exploring metabolic route of several fermentative microorganisms could have been beneficial
7
for process scale up. Thereafter, Kumar et al. reviewed new insight on the development in biobutanol production without considering the essence of bioethanol as one of the major fermentative product of lignocellulosic biomass [31]. Thereafter, an explicit discussion on the transformation of biomass into the selective derivatives of cellulose and lignin will largely cover the entire aspect of lignocellulosic conversion into value-added products [32-34]. The present review discusses on the latest advancements in the process of pretreatment, saccharification and fermentation of lignocellulosic biomass. In view of that, the global
ro of
scenario of biofuel production is discussed based on the initiatives taken by different Governmental agencies. Further, critical analysis on various conversion process during pretreatment are conducted and supported by latest accomplishments. With respect to
-p
mitigate the bottlenecks involved in the process of hydrolysis and fermentation, an insight on the efficacy of microbial community is explored for an improved production of bioethanol
re
and biobutanol as biofuel. Finally, various routes for the conversion of cellulose and lignin
lP
into their derivatives manifests an entire aspect of exploring renewable lignocellulosic biomass as an indispensable option for environmental sustainability. 1.1. Present scenario of biofuel production in India
na
With the present stand in 2019, India has been the fastest economy of the world. Inspite of that, the country still urges to develop sustainable technology in the sector of biofuel to
ur
mitigate the sole dependency on petroleum based fuels. With the aspirations to strengthen
Jo
country’s energy security, planning commission of Government of India released a report in the year of 2003 and mandates regarding 5% ethanol blend with conventional fuel. Further, the next commission increased the mandate to 10% of bioethanol blend for the period from 2007 to next five years. In order to mitigate such long term bottlenecks, Government of India has already issued a recommendation under the agenda of ‘National policy on biofuels’ [35]. The proposal was aimed at minimum level of biofuel makes readily available to meet the
8
energy demand of the country with an aspiration of blending conventional petroleum derived fuel with 20% biofuel including biodiesel and bioethanol by the end of 2030. With regard to international context and national circumstances, the policy is dedicated towards constructing an interdepartmental mechanism to supervise on the developments aimed at more efficient technologies for biofuel production based on indigenous feedstock. In the late 2011, Department of Biotechnology (DBT) under the Ministry of Science and Technology, Government of India ventured a collaborative research work with Indian Oil Corporation
ro of
Limited (IOCL) to set up a steam explosion plant at Centre for Advanced Bioenergy Research in Faridabad, New Delhi. The Centre was fully operative in June 2012 when the pilot plant was installed and commissioned with a collaborative agreement with National
-p
Renewable Energy Laboratory (NREL), USA. The steam explosion pretreatment plant facilitates a through put of 5 KG of dry biomass per hour and subsequent bench scale batch
re
facility for saccharification and fermentation. Recently, as a part of Biofuel mission by
lP
Government of India, Ministry of Science and Technology supported DBT-ICT has started full-fledged demonstration plant for the production of ethanol from lignocellulosic biomass at Centre for energy biosciences at ICT Mumbai with a capacity of 10 tons of biomass/day in
na
April 2016. Likewise, with the prior approval from Ministry of Petroleum and Natural Gas, Bharat Petroleum Corporation Limited (BPCL), is all set to architect India’s first 2G refinery
ur
producing bioethanol at the Bargarh district in Odisha. BPCL is well dedicated to invest
Jo
13,95,76,100 USD to produce 3 crore litre of bioethanol annually with a consumption of around two lakh metric tons of rice straw available locally. Apart from a major thrust, accorded to Research and Development sector, Government of India is to prioritize major innovation based on local feedstock and indigenous technology to attributes towards set up of more pilot scale plants in India. Fig. 2 depicts budgetary allocations implemented through different R&D agencies of Government of India in the recent years, it can be clearly seen that
9
allocations has been increased with the years from 2015 to 2018 manifests Governments’ attempt to boost major research areas in the sector of clean energy through various R&D programme. Maximum fund has been disbursed to MOP followed by DST, MNRE, CSIR, MOPNG and DBT. Table 1 highlights on recent R&D projects on biofuel sanctioned by different agencies of Government of India. From the table it can be comprehended that, around 1,68,936 USD is already sanctioned for various biofuel based R&D projects. 1.2. Scenario on global production of biofuel
ro of
The global demand of biomass based liquid fuel is increasing due to the overrated exploitation of fossil fuels with its subsequent detrimental effect on earth and atmosphere. During the period of 2008 to 2017, the atmospheric CO2 concentration has escalated high into
-p
a level of ~2.3 ppm/year [36]. Fossil fuels attributes towards 80% of primary global energy consumption and out of which ~60% of energy is consumed by the transport sector [37]. As a
re
consequence, of such anthropogenic events, the mean global surface temperature is increased
lP
by 0.85 °C that eventually manifests towards an increased sea level after rapid reduction for ice and snow. With respect to the increase in detrimental gases in atmosphere, EU adopted Kyoto Protocol in the year of 1998, with an aspiration to reduce the emission of greenhouse
na
gasses at or below of 8% during 2008 to 2012. In order to achieve such directives, a number of initiatives were improvised for the better utilization of energy from renewable based
ur
sources. In 2009, renewable energy directives of EU mandated to each of the state’s partner
Jo
towards specific target of 20% of renewable energy consumption along with 20% reduction in greenhouse emissions and 20% increase in energy efficiency by the end of 2020. Since the implementation of the directives, an encouraging 12.5% of total energy consumption is obtained from renewable sources in different states of EU. By 2020, a maximum of 7% first generation biofuel was aimed to introduce in the transport sector. With the revised renewable energy directives of 2016, an agenda was proposed to ensure of at least 27% renewable
10
energy consumption in EU by 2030. Apart from EU, USA has been the world largest producer of biofuel in 2017 with a sole production 16 billion gallon of bioethanol. Since its emergence to support the progress of biofuel as an alternative source of energy over gasoline based fuels, the Government of USA has been keen to provide necessary incentives through various regulations. As a part of implementing the regulation to explore the emergence of biofuel, The Government of USA started with the Energy Tax Act of 1978 [38]. As a consequence, biofuel producers are decided to exclude from paying Federal Gasoline Excise
ro of
Tax with an expectation of producing 10% ethanol blended gasoline. Further, Energy Independence and Security Act of 2007 ensured uninterrupted growth in bioethanol production with futuristic mandates on biofuel usage in USA [39]. Primarily, corn is
-p
considered as main feedstock for bioethanol production in USA with a recorded annual production of 15.1 billion bushels in the year of 2016-17. Following US, Brazil is another
re
leading producer of lignocellulosic ethanol in the world. Brazil is reported to produce 30.7
lP
billion liters of ethanol against the total domestic demand of 28.7 liters in 2018. Utilization of sugarcane as lignocellulosic feedstock attributes towards 61% contribution in bioethanol production of the country. On the contrary, compared to 2017, an increase of 58% was
na
observed in the year of 2018 when ethanol was produced using corn as lignocellulosic feedstock. In an addition, Brazil is well committed towards a mandate of 27% ethanol
ur
blending for gasoline with an aspiration of greenhouse reduction of 37%, 43% by the end of
Jo
2025 and 2030 respectively. Like sugarcane and corn in Brazil, the development of biofuels in China primarily based on an effective utilization of feedstock’s like corn, maize, wheat and rapeseed [40]. Inspite of second largest economy of the world, China’s renewable growth rate is considered insignificant, as the contribution of biofuel remains low in the sector of bioenergy. However, the country’s ambition of reducing greenhouse gas emission becomes more realistic when 82% growth in renewable energy was accounted compared to only 16%
11
growth in the consumption of fossil fuels during 2010-2015 [41]. As of present time, China has completely transformed its status from producing zero biofuel into the fourth largest producer in world. As a result, an amount of 9,770 million liters of ethanol production is forecasted in the year of 2018. Nevertheless, Chinas’ long-term aspiration directs near to fivefold escalation in ethanol consumption from the present is severely interrupted due to country’s environmental policies and strict economic regulation.
2. Conventional pretreatment methods
ro of
2.1. Pretreatment with aid of physical methods
Physical pretreatment is aimed to impose mechanical forces on biomass in terms of impact, friction, shear and compression. Most of the major mechanical forces are generated from the
-p
impact and friction within biomass particles [42]. A sieve or screen allows the control on the particle size of desired product. Based on the particle size of the desired product, cutting
re
strategies are subcategorized into crushing (meter to centimeter), milling (cm to 500 µm),
lP
micronization (cm to 100 µm), fine grinding (less than 100 µm), ultra-fine grinding (less than 30 µm) [43]. A number of different tools such as ball mill and extruder cut the biomass into
na
different fragments in order to achieve the desired particle size as the product. During ball mill operation, biomass undergoes impact and compression stress when collision occurs with
ur
ball and wall of the reactor. Due to the generated stress within the ball mill, deconstruction of biomass attributes towards more accessible area readily available for subsequent hydrolysis
Jo
process.
12
2.1.1. Pretreatment using ball milling Primarily, ball milling is conducted with biomass and ceramic balls together in a mill at a specific rotation represented as rpm. The collision between biomass particle with the balls at a fixed rotation leads to break the crosslinks in between cellulosic chain that attributes to a disruptive structure resulting in more porous formation within the surface of biomass [44]. In view of that, ball mill generally uses ball made up of metal or ceramic to grind up the biomass particles. Based on the movement of the balls within the mill, ball mills are
ro of
classified into conventional, attrition and planetary mill. Conventional mill utilizes the natural force of gravity on the biomass particles, planetary mills generated artificial force from centripetal movement inside the mill and attrition mills rotates the rotor to stir the grinding
-p
balls. Due to low efficiency, conventional ball mills are mostly used in a combination with chemical pretreatment. In case of planetary mill, milling resulted from the movement of
re
rotating bowl and the balls whereas, milling is generated from the stirring action of rotating
lP
impeller with the arms for attrition mills. Inspite of high energy demand, ball mills possess a promising approach to reduce particle size and degree of crystallinity of biomass. Lu et al. investigated that the change in particle size with milling time and at the initial phase of
na
milling an exponential decrease in the median diameter of cellulose particles attributes towards a maximum 47% of decrease followed by an insignificant change in particle size at
ur
further increase in milling time. Also, after 60 min of milling period, specific surface area
Jo
was observed to increase to a maximum of 2.25 m2/g compared to a surface area of 1.37 m2/g of native cellulose [45]. Further, the ball mill facilitates ~23% reduction in degree of polymerization (DP) of biomass after 40 min of continuous rotation. Thus, the decrease in DP is due to the cleavage of β-(1-4) glycosidic linkages in linear cellulosic chain manifests the severity of ball milling pretreatment in disrupting polymeric nature of cellulose. Other than rotational speed, water content and reaction temperature are the critical factors in determining
13
the effect ball milling pretreatment on biomass. Gu et al. observed fast grinding of corn stover when milling was conducted without of using any water in the reaction mixture. A maximum 67.0% (w/w) of glucose was produced when corn stover was milled in the absence of water at 80 °C for 30 min. [46]. As water was studied to show significant effect on biomass hydrolysis during ball milling technique. Hence, in an another study, the effect of ball milling technique was intensified using combined pretreatment of biomass and a mixture of solvents (water, ethanol, sulphuric acid and hydrogen peroxide) at varying temperatures in
ro of
the range of 100 to 150 °C with an interval of 10 °C. It was noticed that, xylose was the only sugars obtained from corn straw at lowest temperature of 100 °C (without ball milled). However, glucose was released only when ball milled sample was reacted at or above 120 °C
-p
[47]. Thereafter, an insignificant increase in sugars yield was observed with increase in temperature for ball milled sample. Table 2 represents the effect of ball mill pretreatment on
re
different lignocellulosic biomass. From the table it can be observed that, combined treatment
lP
primarily preceded by ball milling resulted into more severe disruption in the structure of biomass. Beside a number of advantages, high energy input during the process has been a
na
major challenge towards the practical applicability of ball mill pretreatment, which demands to address the issues dependent on; (i) rotation speed of the motor, (ii) capacity of ball mill,
ur
(iii) structure and properties of native biomass and (iv) desired particle size. Among different advantages, reduction in cellulose crystallinity and degree of polymerization of biomass are
Jo
most important outcome whereas, high power input during ball milling slows down the practical applicability of the process. 2.1.2. Pretreatment using extrusion After ball milling, extrusion has been another promising pretreatment method which involves heating, mixing and shearing of biomass that primarily brings about the physical and chemical alterations attributes to significant exposure of cellulose towards subsequent 14
hydrolysis process. The sole efficiency of this process to operate at high solid loading advances its effect when combined with other chemical methods. During the process of extrusion, moderate operating conditions within the reactor (known as extruder) facilitates less formation of inhibitory by products. A number of research studies on extrusion pretreatment hypothesized the fact that, the variation in sugar yield largely depends on the process condition and nature of biomass used. Table 3 summarizes different operating conditions for different biomass to improve the process of extrusion. The table shows
ro of
moisture present in the biomass, temperature of the reaction media, screw speed and screw elements are essential for an effective extrusion process attributed to substantial decrystallization and delignification of biomass. A maximum of 69.5% delignification was
Extrusion process and configuration of extruder
-p
obtained for corn stover.
re
Extrusion has been a widely acknowledged thermo-mechanical pretreatment process based on
lP
the shear force exerted by the rotating screws on the biomass within the extruder. The reactor is consisted of screws rotate at a specific rotation using motor within a barrel. Single screw extruders are constructed with single solid screw within barrel of extruder while twin screw
na
extruders are architected with multiple screw-like elements assembled over shaft. Based on the type of the screw material used, twin screw extruders are classified as co-rotating and
ur
counter-rotating extruders and the biomass is feed through the hopper section. Fig. 3 shows
Jo
the schematic of a conventional twin screw extruder. The extruder is configured with a hopper for the collection of sample biomass. Motor supplies the energy to rotate both the screws. The barrel mainly represents the reaction zone which is thermally regulated. Due to the differences in the configuration of screw elements, the operation of extruders largely differs in the way the biomass is mixed and flows through the barrel. Compared to single screw extruder, twin screw is observed to produce more intense mixing and reduction in
15
particle size thus attributes towards more rigorous changes in the physical properties of biomass. Apart from the good mixing properties, counter rotating screws are advantageous because of self-cleaning ability. Factors influence the process of extrusion Like the other pretreatment processes, extrusion also depends on a number of interrelated parametric conditions. Among all, screw profile (configuration, rotation), temperature and solid loading inside the reactor mostly influence the extrusion process.
ro of
Screw profile and speed used in extruder The change in the properties of biomass depends on the mechanical force generated due to the variations involved in the screw configurations. Apart from the mechanical forces, screw
-p
profile is also considered on a high extent to fixed up other parametric variables such as solid loading. Further, basic arrangements of single screw or twin screw extruders demand more
re
incorporation of associated elements to advance the process of mixing. Among various
lP
orientations in the configuration of screws, application of reverse kneading disk was appeared to exert maximum pressure to disintegrate the hardwood (Angiosperm trees produced hardwood biomass and seeds of this pants are mostly covered) biomass effectively [48].
na
Additionally, incorporation of reverse elements in the screw improves residence time of the biomass thus excel cellulose digestibility with reduction in crystallinity [49]. Further,
ur
residence time of the biomass highly depends on screw speed. With a fixed intake of
Jo
biomass, residence time decreases with increase in screw speed. Most of the extrusion pretreatments are confined within low rotation in screw speed to increase the residence time. In view of that, optimum screw speed of 7 Hz was obtained for twin screw extruder when wheat bran was used as lignocellulosic biomass and in the same experiment, Lamsal et al. identified optimum screw speed of only 3 Hz when soybean hulls were used [50]. Karunanithy et al. used only 1.7 Hz as optimum screw speed in single screw extruder for the
16
pretreatment of switch grass [51]. Apart from screw profile and speed, temperature of the extruder has been an another dictating factor associated with the process of extrusion. Temperature Temperature within the extruder has been an important parameter as it directly impacts on the hydrolysis of biomass. However, at high temperature possible condensation of lignin frequently limit the rate of biomass hydrolysis. Sometimes, bio char is also formed from excessive torrefaction of biomass at high temperature [49]. Further, presence of catalyst
ro of
reduces the required temperature and it was often noticed that, extrusion carried out at acidic pH necessitates high temperature [52]. Hence, low temperature has been quite imperative at low pH conditions to avoid the formation of degradation products from sugars or lignin of
-p
biomass. Liquid to solid ratio of biomass
re
The aspect ratio of liquid to solid refers the amount of liquid added into the reaction mixture
lP
which is required to maintain the flow during the process of extrusion. Addition of liquid leads to soften the polymeric cellulose and subsequently breaks the network of microfibril bound. Though, excessive addition of water need to be restricted to avoid over dilution of the
na
sample produced by extrusion pretreatment. During non-catalyzed reactions, the moisture content present within the biomass largely impact on the development of shear forces
ur
resulting in substantial effect on biomass disintegration. The formation of thin film due to
Jo
over absorption of moisture by corn stover substantially reduce the effect of wet ball mill pretreatment [53]. Limited sugars degradation and high continuous throughput are the merits of extrusion process employed for the pretreatment of biomass prior to hydrolysis and highenergy input and instrumental complexities are the subsequent limitations of this technology.
17
2.1.3. Pretreatment with the aid of irradiation The physical pretreatment processes so far discussed are milling and extrusion primarily concerned to increase the surface area of biomass at an expense of high amount of energy consumption. Therefore, an alternative of using irradiation in the form of gamma ray or electron beam has emerged as a potential replacement. On an addition, radiation technique has been well established to effectively disintegrate cellulose for most of the lignocellulosic biomass [54-56]. Further, the process of irradiation is not associated with the requirement of
ro of
high temperature and no inhibitors are normally produced in the hydrolysate as a result of the irradiation pretreatment. Though, to increase the effectivity, irradiation treatment is frequently performed in combination with chemical treatment [57, 58]. During the combined
-p
treatment, initially irradiation alters the physical structure of biomass followed by the penetration of the chemical agent into the biomass. Lee et al. irradiated kenaf core biomass
re
with 500 kGy of electron beam and following acid treatment with 3% (v/v) sulphuric acid,
lP
the biomass produced a maximum of 73.6% sugar yield after 72 h of enzymatic hydrolysis [57]. Furthermore, the effect of the sequence followed during two stage pretreatment of irradiation and chemical addition substantially improves the sugars yield. Yin et al. showed
na
that, the performance of wheat straw irradiated then swallowed with 4% (w/v) sodium hydroxide was comparatively better than chemical soaking before gamma ray irradiation.
ur
During this investigation, the ability of irradiation to reduce alkali consumption with time
Jo
was noticed when 72.7% (w/w) of reducing sugar was obtained after an hour at 100 kGy of irradiation and in the presence of 2% (w/v) of sodium hydroxide [58]. According to them, the effect of irradiation on substantial lignin removal significantly exposed the cellulosic structure towards sodium hydroxide swelling. However, in an another experiment conducted by Kapoor et al. showed that during the irradiation pretreatment of sugarcane bagasse, gamma ray merely produced any significant changes in the lignin content of pretreated
18
biomass [59]. Further, compared to raw biomass, pretreated biomass showed comparable cellulose content when treated under low dosage of irradiation. Thereafter, significant decrease in cellulose content was noticed at each of the individual increase in irradiation dosages. Further, the impacts of different irradiation methods during the pretreatment of biomass greatly varies from one biomass to another. Hence, Table 4 summarizes an assessment of irradiation pretreatment on different lignocellulosic biomass. Different parameter of the biomass like composition of cellulose, hemicellulose, lignin, crystallinity,
ro of
particle size is primarily affected due to the irradiation pretreatment. Irradiation dose has a significant role in the change of cellulose crystallinity for different lignocellulosic biomass.
2.2. Pretreatment with aid of chemicals
-p
2.2.1. Pretreatment of lignocellulosic biomass using acid
Cellulosic chains of lignocellulosic biomass are well connected through β-(1-4) glycosidic
re
bonds of repetitive glucose units. During the pretreatment of biomass using acids, hydrogen
lP
ion attacks the glycosidic oxygen and subsequently breaks the monomeric sugar molecules apart from polymeric chain. Hence, introduction of acids during the pretreatment technology
na
appeared as an effective process to covert the biomass into value-added products. In view of that, a number of inorganic acids such as sulphuric acid, hydrochloric acid, nitric acid,
ur
phosphoric acid are mostly applied on a variety of lignocellulosic biomass to further improve the production of sugars [60, 61]. During the acidic interaction, hemicellulose fraction of
Jo
biomass is partially hydrolyzed and part of the lignin is slightly solubilized [11, 62]. It has been observed that acid concentration and reaction period are the two most important factors during acid pretreatment process. During the reaction, application of diluted acid is sometimes preferred to substantially produce oligomers of xylose and glucose with a comparative production of monomeric sugars (glucose and xylose) at longer residence time. Nonetheless, longer residence time leads to degrade the cellulosic and hemicellulosic derived 19
sugars into organic acids and furans. Further, diluted sulphuric acid at a concentration of 0.05% (w/w) showed severe degradation efficiency of sugars into organic acids and furans compared to 0.01% (w/w) of sulphuric acid at a comparable reaction condition during two stage acid treatment of cassava [63]. In an another approach of fractional hydrolysis by Mishra et al., four different inorganic acids at varying concentrations were tested on kans grass biomass for maximum fermentable sugar recovery with minimum toxicity. Among the acids, HNO3 recovered a highest of 84.3% (w/w) glucose and 89.7% (w/w) of xylose.
ro of
However, pretreatment with sulphuric acid resulted in an average of 1.3 times less formation of furfural and phenolic compounds compared to HNO3 [61]. Pretreatment using hydrochloric acids in combination with other techniques has been observed to improve the sugar yield
-p
quite efficiently. In an addition hydrochloric acid requires less temperature than sulphuric acid thus reducing the chance towards the formation of inhibitory by products from the
re
degradation of sugars [64]. Sole efficacy of using 1% (w/w) of hydrochloric acid during the
lP
pretreatment of corn stover for 40 min produced 1.6 times more glucose yield than the native biomass after enzymatic hydrolysis. Also, glucose yield reached a maximum of 71.5% when HCl pretreatment of biomass was immediately followed by ammonia wet oxidation
na
pretreatment at 120 °C temperature for 40 min [65]. In another experiment by Chen et al. the sequential investigation of hydrochloric acid and ionic liquid pretreatment of poplar wood
ur
substantially increased cellulose accessibility and finally leads to a maximum fermentable
Jo
sugars yield of 87.2% after 72 h of enzymatic hydrolysis [66]. Apart from sulphuric, nitric and hydrochloric acids, pretreatment with acetic, citric, oxalic acids have also been observed to effectively increase biomass hydrolysis. Rattanaporn et al. reported that, 10% (w/w) acetic acid significantly improved the saccharification of oil palm shell at a temperature of 100 °C for 30 min. Compared to untreated one, acetic acid treated oil palm shell produced a maximum of 40.4 mg of reducing sugars/g of biomass [67]. In an another experiment mild
20
acetic acid concentration of 3, 7 and 11 g/L was used for the pretreatment of switch grass and at 170 °C for 20 min of reaction period, the residual glucan content has been increased at an average of 33.1% than untreated biomass [68]. Additionally, citric acid was also reported as an effective pretreatment agent specially for algal species. During citric acid catalyzed pretreatment of macro algae Gracilaria verrucosa, 0.1 M citric acid was able to produce 51% (w/w) of total reducing sugars at 150 °C for 60 min [69]. In addition to acetic acid and citric acid, oxalic acid also solubilizes the hemicellulose fraction of biomass on a great extent. The
ro of
severity of the pretreatment was reported at 82 M of acid concentration at 160 °C for 58 min when oxalic acid was incorporated with yellow poplar biomass [70]. Due to a rapid degradation of xylan content, crystallinity of pretreated biomass was increased till a certain
-p
pretreatment severity.
During acid pretreatment, a number of unwanted chemicals are produced in the reaction
re
mixture known as inhibitors for fermentation process. Under protonated condition and at
lP
prolonged reaction period, the produced monomeric sugars liberated three molecules of water for the formation of furan compounds. Furfural is generated as furan from five carbon
na
pentose sugars like xylose and arabinose whereas, hydroxymethyl furfural is produced from six carbon hexose sugars like glucose, galactose and mannose [71]. From furfural, acetic acid is produced and from hydroxymethyl furfural, both of the acetic acid and levulinic acid can
ur
be produced. Apart from furfural and hydroxymethyl furfural, acetic acid is also produced
Jo
from the acidic hydrolysis of xylan backbone of hemicellulose. Detoxification of inhibitors mandates an elimination of theses inhibitors from the reaction mixture prior to the process of fermentation. Short residence time and high production of sugars are the advantages of using acid pretreatment of biomass and dis-advantages include significant removal of hemicellulose and formation of unwanted inhibitors in the reaction mixture.
21
2.2.2. Pretreatment of lignocellulosic biomass using alkali Pretreatment with the incorporation of alkali facilitates substantial lignin removal and subsequently manifests towards more porous structure formation. Besides, alkaline pretreatment improves the accessibility of cellulose by disrupting a certain portion of the biomass. Yang et al. studied the effect of alkali on delignification of bamboo in a combined treatment of hot water and sodium hydroxide at a range of 0.5-2% (w/v) [72]. It was observed that, compared to raw biomass, content of lignin was substantially decreased with each of the
ro of
increase in alkaline concentration and temperature of the reaction. Still, in some occasions the residual lignin in the pretreated biomass impaired the hydrolytic efficiency of biomass. Li et al. observed that, recovery of cellulose and hemicellulose reduced after 70 °C temperature
-p
when sugarcane bagasse was pretreated with NaOH and in the presence of H2O2 as an oxidative agent [73]. Besides, significant lignin removal of ~65% was observed for sugarcane
re
bagasse when the reaction temperature was at fixed at 50 °C followed by an insignificant
lP
lignin removal with further increase in temperature. Further an optimum lignin removal of around 70% was observed at 1% (w/v) of alkaline concentration. Hence, an efficient process of pretreatment is evaluated on the basis of removal of lignin from the biomass which
na
attributes towards an enhancement in the cellulosic conversion. In view of an increased lignin removal a sequential acid and alkali treatment was implied on A. salmiana leaves and a post
ur
acid pretreatment was revealed with significant hemicellulose solubilization. Compared to
Jo
biomass, an increase of ~137% in lignin removal was reported when acid pretreated biomass was subjected to 3.4% (w/v) of NaOH pretreated for 70 min [74]. Effective removal of lignin as a result of alkaline pretreatment makes cellulose and hemicellulose accessible towards enzymatic hydrolysis and attributes towards enhanced sugar production. However, a large proportion of biomass solubilization is observed as major limitation of the process.
22
2.2.3. Pretreatment of lignocellulosic biomass using ionic liquid Pretreatment with ionic liquids has been a major attention in the field of lignocellulosic research as the reaction medium is consisted of organic cation and inorganic anion or cation. Most of the investigations since its exploration, ionic liquids have been observed to showcase few of its tremendous properties such as high thermal stability, less vapor pressure, nonflammability [75]. Due to the low vapor pressure, ionic liquids limit the emittance of volatile components hence considered as green solvents. The potential of various ions presents in
ro of
ionic liquids manifested an effective attempt to dissolute cellulose and solubilize most of the hemicellulose. Ionic liquids dedicated to interact with hydroxyl groups of cellulose are often studied to disrupt the intra and intermolecular hydrogen bonds [76]. Among different ions,
-p
ionic liquid consisted of pyridinium and imidazolium is proven as one of the suitable compounds to efficiently dissolve cellulose. Sashina et al. observed the superiority of 1, 3-
re
disubstituted pyridinium salt as an efficient cellulose dissolution medium over, 1,2-
lP
disubstituted pyridinium. [77]. Saher et al. studied the ability of 1-butyl-3-methyl pyrodinium chloride to effectively dissolve 28% cellulose at 110 ºC [78]. Trinh et al. in an another study of pretreatment revealed that, the potential of 1-butyl-3-methylimidazolium chloride towards
na
significant cellulose digestibility with a fold increase of 29 and 20 for hard wood and soft wood respectively [79]. Miranda et al. addressed the efficiency of different protic ionic
ur
liquids to facilitate lignin removal of pine apple crown and it was studied that bis-2-
Jo
hydroxyethyl ammonium propionate attributed towards 90% of lignin removal in the pretreated biomass [80]. Hence, pretreatment of biomass using ionic liquids has been considered as green technology with respect to biomass dissolution towards subsequent accessibility for enzymatic hydrolysis. Several advantages of ionic liquid pretreatment are effective dissolution and significant delignification of biomass on the contrary, separation of
23
desired product and detoxification of the inhibitors are the major disadvantages of the process. 2.2.4. Pretreatment of lignocellulosic biomass using organic solvents Organosolv pretreatment has been considered as another promising approach of biomass fractionation mainly focused on delignification. On an addition, removal of lignin with satisfactory solubilization of hemicellulose attributes towards more effective accessible area within the biomass for subsequent hydrolysis process. Substantial disintegration of biomass
ro of
depends on type of solvent used. Organosolv treatment employs a number of organic solvents such as ethanol and methanol. During the interaction with the biomass, degradation of lignin manifests towards breakdown of α and β aryl of ether linkages in one type of reactions
-p
followed by the disruption of glycosidic bonds in major part of hemicellulose and cellulose for the conversion into oligosaccharides [81]. Over the recent few years’ deep eutectic
re
solvent has emerged as potential delignification medium due to its high biocompatibility and
lP
bio sustainability [82]. Also, it is highly considered as a perfect alternative for ionic liquids. Further, research focus is more likely inclined towards limiting high viscosity of eutectic solvents. In view of diluting the solvents, New et al. studied, the effect of water in the
na
preparation of eutectic solvent as choline chloride: urea at 1:2 ratios during delignification of oil palm fronds and 30 % (v/v) was found optimum to conduct 16.3% of delignification [83].
ur
In an another experiment, Yu et al. developed a modified hot water pretreatment process
Jo
based on water: choline chloride at 2:1 ratio which leads to a maximum lignin removal of 53.6% from leave sheaths with a minimum consumption of solvents [84]. Chen et al. synthesized an organic solvent mixture of various hydrogen bond donors and acceptors and it was evidenced that, the mixture of guanidine hydrochloride as H-bond acceptor with ethylene glycol and p-tolunesulfonic acid as H-bond donor was the most effective to remove 80% of xylan and lignin from switch grass pretreated at 120 ºC only for 6 min of reaction period [85].
24
So, pretreatment of biomass using eutectic solvents as organosolv has advanced a mean of green technologies for sustainable disintegration of the constituents of biomass. Pretreatment using organic solvent is studied as an effective strategy for both hardwood and softwood with high product yield at an introduction of acid. Requirement of an extra streamline for recovery of chemicals, high process cost are the disadvantages. 2.3. Physico-chemical pretreatment Physico-chemical pretreatment refers to the application of high pressure and temperature to
ro of
the biomass followed by sudden release of the pressure which brings about structural changes through the disruption of intra and intermolecular linkages. Among physico-chemical pretreatments, steam explosion, liquid hot water and wet oxidation are mostly investigated.
-p
2.3.1. Steam explosion pretreatment
Steam explosion has one of the crucial pretreatment technology to promote the breakdown of
re
lignocellulosic matrix at high pressure in the range of 1 to 5 MPa of pressure generated by
lP
saturated steam and high temperature in the range of 150 to 300 ºC within the reactor for a short span of time [86, 87]. As a result of severe pretreatment conditions, substantial disruption of biomass matrix attributes towards partial removal of lignin and the process like
na
auto-hydrolysis mediating through the solubilization of hemicellulosic sugars. Partial removal of lignin manifested towards the formation of pseudo lignin over biomass surface. So, in view
ur
of reducing the lignin content, addition of chemicals facilitates the process of substantial
Jo
delignification during steam explosion pretreatment of biomass. Veradi et al. observed that the efficiency of steam explosion pretreatment was significantly improved when sugar cane biomass was impregnated with 0.2% and 1% (v/v) of hydrogen peroxide solution and an average increase of 12% and 34% in glucose yield from H2O2 impregnated biomass was observed compared to untreated one [88]. In an another steam explosion pretreatment, 0.5% (w/w) H2SO4 was impregnated on elephant grass and at optimized reaction temperature of
25
161 ºC, the treated biomass was obtained with 52.1% (w/w) of cellulose after 11.5 min of time which finally attributes towards 55% of scarification yield after enzymatic hydrolysis [89]. Wang et al. also reported substantial presence of monosaccharides in the pretreated filtrate when spruce was impregnated with both of sulphuric acid and sulphur-di-oxide. Pretreated filtrate obtained from biomass using both of the impregnating reagent resulted into significant inhibition towards cellulolytic enzymes during enzymatic hydrolysis [90]. Steam explosion treatment is observed as effective against hardwood and agricultural residues.
ro of
However, high hemicellulose removal and ineffectiveness against softwood biomass impedes the technology from practical application. 2.3.2. Liquid hot water pretreatment
-p
Liquid hot water also designated as hydrothermal pretreatment has been well known as green technology to disintegrate the intricate structural matrix of biomass. Furthermore, the process
re
involves compressed water as the solvent with no extraneous addition of any chemicals. So,
lP
the evolved energy and cost of the process is comparatively less and the feasibility towards the formation of inhibitors is less. Moreover, selective hydrolysis of hemicellulose mediating through the formation of hydronium ions partially interfere with the alterations of
na
lignocellulose and the formation of pseudo lignin with unwanted inhibitors vastly disturb the process of enzymatic hydrolysis. On an addition, even at the higher temperature of 200 ºC,
ur
reallocation of lignin was vigorously reported on the surface of biomass fibers manifested
Jo
towards more residual lignin. Consequently, different attempts are made to determine the optimized hydrothermal condition for different biomass. Batista et al. hydrothermally pretreated sugarcane straw at different reaction times and temperatures and the severity of the reaction for temperature is observed as dominant over reaction time. At optimized reaction condition of 195 ºC for 10 min, 58.8% (w/w) of cellulose was obtained in the pretreated biomass compared to the existence of 33.1% (w/w) of cellulose in the raw sugarcane straw
26
and further 85.5% (w/w) of hemicellulosic sugars are also reported in the pretreated filtrate [91]. Haldar et al. reported the potential of hot water pretreatment to prepare the feedstock obtained from waste banana stem for enzymatic hydrolysis and an incorporation of 20 FPU cellulase enzyme resulted into the production of 12.8 g/L of total reducing sugars [92]. Lyu et al. conducted two stage liquid hot water pretreatment using cassava straw as lignocellulosic biomass and at the optimized reaction conditions (180 °C for 60 min in first stage and 200 °C for 20 min) ~83.1% (w/w) of pentose sugars was obtained with a substantial yield of ~85%
ro of
(w/w) of hexose sugars from enzymatic hydrolysis [93]. Hence, liquid hot water treatment facilitates towards maximum hemicellulose removal with substantial cellulose recovery in the residual biomass and also attributes towards less formation of inhibitory compounds as side
-p
product. The advantages of liquid hot water treatment are no requirement of adding any external chemicals and maximum portion of lignocellulosic contents are retained in the
re
pretreated biomass whereas, the disadvantage is an insignificant removal of lignin from both
lP
of hardwood and softwood. 2.3.3. Wet oxidation pretreatment
Wet oxidation has been another physicochemical pretreatment method in which biomass is
na
treated in the presence of oxygen or air as an oxidant and at the temperature in the range of 100 to 200 °C. The applied pressure is varied up to a maximum of 20 MPa for residence time
ur
in the range of 0.5 to 2 h [94]. Moreover, instead of oxygen, incorporation of other chemical
Jo
as oxidant leads the process cost effective. Therefore, uses of oxygen or air as an oxidant is mostly preferred during any wet oxidation treatment. During the treatment, breaking down of the solid crystalline matrix of cellulose facilitates hemicellulose conversion from solid polymeric phase to liquid oligomeric phase. Balat et al. showed that, at temperature more than 170 °C, lignocellulosic biomass undergoes autohydrolysis and attributes towards formation of organic acids which leads to bring down the pH of the medium in the range of 3
27
to 4.5 [95]. However, more acidic pH of the reaction medium leads to produce hydroxyl methyl furfural and basic medium leads to produce aldehyde compounds [96]. Table 5 highlights on the recent advancements of different physicochemical pretreatment methods for various lignocellulosic residues. The table highlights on the mostly explored lignocellulosic biomass such as rice straw, wheat straw, sugarcane bagasse, corn stover and sorghum bagasse and it can be briefed that, severity of the effect of pretreatment depends on the nature and type of the biomass. Biomass treated with alkali or ionic liquid shows substantial
ro of
improvement in sugar yield. Efficient removal of lignin and low formation of inhibitors are the merits of wet oxidation of biomass while high cost involved with oxygen and catalyst limit the process from its ground applicability.
-p
2.4. Biological pretreatment
Most of the pretreatment processes discussed earlier are associated with high energy
re
consumption or economically unsuitable. However, less energy input and minimum
lP
requirement of chemicals established biological pretreatment process as a sustainable alternative for lignocellulosic disruption. In view of lignocellulosic disintegration, lignin has been an essential constituent of biomass as it heavily cross linked within the intricate network
na
of cellulose and hemicellulose. A number of white rot fungi have been discovered with potential ability to breakdown the lignin of the biomass through the production of ligninolytic
ur
enzymes. Deswal et al. studied the maximum production of ligninolytic enzymes at fifteen
Jo
days of solid state fermentation when white rot fungi like P. florida, C. caperata and G. sp. were inoculated into the reaction mixture of sugarcane bagasse. Besides, a highest delignification of 7.9% was obtained for P. florida after 15 days of solid state fermentation. In an another study of biological pretreatment with different biomass, a white rot fungus I. lacteus was observed to bring about highest lignin removal of 42.3% and 45.8% for wheat straw and corn stover respectively [97]. Martinez Patino et al. studied biological treatment of
28
olive tree biomass and compared to raw untreated biomass, a significant increase of 312.5% was observed in glucose concentration after enzymatic hydrolysis of the biomass obtained after a sequential 45 days of continuous pretreatment in the presence of I. lacteus followed by acid hydrolysis with 2% (w/v) H2SO4 [98]. Under the condition, enzymatic hydrolysis reached with a yield of around 50% along with glucose recover of 39.6%. Among the advantages, an effective degradation of lignin and partial removal of hemicellulose at mild reaction conditions manifests less energy consumption whereas, slow reaction rate and
ro of
unsatisfactory yield compared to the other available process stands as barrier for biological pretreatment process
Pretreatment is regarded as a crucial unit operation in order to break the outermost shell of
-p
lignin for making cellulose and hemicellulose accessible to enzymatic hydrolysis process. In view of that, pretreatment of biomass using hot water may be a promising approach for the
re
biomass with low lignin content as the process doesn’t demand of any extraneous addition of
lP
chemicals to disrupt the structural intricacy of lignocellulosic residues. However, alkaline treatment facilitates substantial delignification of the biomass with high content of lignin. 2.5. An insight on latest and advanced emerging pretreatment technologies
na
The conventional methods of pretreatment processes described in preceding section is focused primarily on the breakage of the biomass recalcitrance. However, these techniques
ur
suffers severely from an effective and practical solution with regard to an industrial
Jo
adaptation [99]. Interestingly in the recent years, with the advent of applied research, a number of promising approaches are emerged as green technologies for the pretreatment of lignocellulosic biomass. Among the others, few of the emerging technologies involve an irradiation of microwave, electron beam and gamma ray along with the application of pulsed electric field and high hydrostatic pressure as advanced form of pretreatment methods for biomass disintegration. During pretreatment, an electromagnetic energy of microwave
29
converts well into heat and dissipated uniformly throughout at the molecular level within the biomass particles manifests consistent disruption as an effect of microwave irradiation. Recently, microwave assisted alkali treatment with 0.5% (w/v) of NaOH was reported as the best suited method for pretreatment of brewers spent grain when 1 g of biomass was treated at 400 W only for 60s leads to the production of 20% (w/w) of reducing sugars manifests ~3 fold increase compared to an untreated biomass [100]. During electron beam irradiation, ionizing radiation is generated from linear accelerator for the formation of free radicals, once
ro of
the biomass is exposed into the reaction system. In view of that, 500 kGy of electron beam was used to depolymerize M. sinensis for an efficient production of fermentable sugars [101]. Application of pulsed electric field facilitates delignification and porous formation within the
-p
surface of biomass attributes towards an easy penetration of hydrolytic enzymes through the pores. Kumar et al. studied the effectiveness of pulsed-electric field when wood chip and
re
switch grass was pretreated under the influence of 2000 pulses and at field strength of 10
lP
Kv/cm resulted into an enhanced conversion of biomass into value-added products [102]. The concept of introducing high hydrostatic pressure in the range of 100 to 600 MPa on lignocellulosic biomass is subjected with an aspiration of uniform distribution of pressure and
na
as a result, 5-10 fold increase in initial rate of xylan hydrolysis was observed when E. globulus pulp was exposed towards 300-400 MPa hydrostatic pressure for 45 min [103].
ur
Raud et al. studied an effective and novel decompression pretreatment method for
Jo
lignocellulosic biomass under nitrogen explosion in the range of 1-3 MPa at 150 °C with no addition of any extraneous chemicals [104]. Further, Ravindran et al. investigated another novel FeCl3 assisted plasma pretreatment for spent coffee waste and as a result, 0.496 g of reducing sugars per g of treated biomass attributed into the production of 18.6 g/L of ethanol when the hydrolysate was fermented in the presence of S. cerevisiae [105]. The emerging pretreatment methods are considered as sustainable approaches for subsequent hydrolysis of
30
lignocellulosic biomass due to the promising ability of the technologies to offer maximum operational output at an expense of minimum efforts. The advancement in lignocellulosic biomass hydrolysis is discussed in details in subsequent sections.
3. Strategies towards improved enzymatic hydrolysis of biomass On an economical perspective, enzymatic hydrolysis has been a deciding factor towards an efficient conversion of biomass to biofuel. Following a number of various pretreatments, the solid biomass is subjected to rigorous enzymatic hydrolysis in the presence of cellulase,
ro of
hemicellulase and β glucosidase enzymes [106]. Microbial enzymes extracted from different microorganisms have been experimented on a variety of biomass to obtain the maximum yield in an optimum reaction condition. Further, enzymatic hydrolysis is mostly preferred as
-p
the process is not associated with the toxic productions of inhibitors thereby save the accessory cost including detoxifications of inhibitors. Moreover, high substrate concentration
re
facilitates towards better yield with an expense of more enzyme consumption [107].
lP
Nevertheless, incorporation of high dose of enzyme excels the cost of the process and hamper the sustainable production of biofuels. As a result, selection of proper enzyme cocktail with
na
an identification of optimized reaction condition has been a challenging aspect without compromising the product yield. Table 6 explains the effectiveness of using enzyme cocktail
ur
towards an improved enzymatic hydrolysis. From the table, it can be said that when cellulase enzyme was provided with other accessory enzymes then the enzyme cocktail shows
Jo
substantial improvement in the hydrolysis of biomass. Enzymatic hydrolysis is mostly conducted with cellulase enzyme extracted from Trichoderma reesei [108-110]. Deficiency of β-glucosidase activity in cellulase enzyme increased the accumulation of cellobiose attributes towards product inhibition. Hence, incorporation of β-glucosidase enzyme with cellulase enzyme improved the product yield obtained from enzymatic hydrolysis of biomass [111, 112]. In order to compensate with high cost of enzymes, different strategies were made 31
to improve the saccharification yield. In view of that, addition of additives such as Triton X100 and Tween 20 have been exclusively used and showed an increase in the hydrolysis rate by improving enzyme stability and activity [113]. Martin et al. studied the effect of polyethylene glycol on the hydrolysis of pretreated biomass and subsequent study revealed that, an average of 8.8% increase in glucose yield was observed when substrate was corn stover and sugarcane straw with 32% increase for avicel [114]. Likewise, in an another study alkalophilic fungus MVI2011 was developed to produce substantial ligninase enyme for a
ro of
rapid disintegration of rice straw and subsequent study on submerged cultivation for seven consecutive days’ attributes towards 74.2% increase in saccharification yield. Further, from compositional aspect, as glucan and xylan are the major polysaccharides present in the
-p
biomass so integrated activity of cellulase and xylanse has been beneficial towards an effective hydrolytic conversion. Chen et al. developed a bi-functional enzyme
re
cellulase/xylanase (CtCel5E) derived from C. thermocellum and fused with β-glucosidase
lP
(CcBgIA) from C. cellulovorans which resulted into an improved production of glucose from carboxymethyl cellulose and rice straw [111]. During enzymatic hydrolysis, a prompt release of product is naturally obtained at very starting phase of hydrolysis which was followed by a
na
sluggish rate in production. The decrease in the production manifests towards two different possibilities. One, consumption of substrates with the reaction leads to decrease the
ur
production and other is the accumulation of products, interfere with the action of enzymes
Jo
and process of hydrolysis. Haldar et al. studied the inhibitory effect of different monosaccharides (glucose, xylose, galactose, mannose and arabinose) disaccharide (cellobiose) and inhibitors (furfural, acetic acid and HMF) through the external additions into the reaction mixtures during enzymatic hydrolysis of waste banana stem and it was observed that, apart from arabinose, each of the external additions leads to inhibit the enzymatic system of cellulase commercialized from T. reesei [92]. Hence, continuous monitoring of the
32
concentration of sugars in the hydrolysate provides an insight to tackle the product inhibition. In an enzymatic inhibitory concept, other than product, substrate inhibition is an important factor to analyze the decrease in the production. More substrate leads to produce more sugars in any conventional enzymatic system. However, during enzymatic hydrolysis, the increase in production with the increase in substrate concentration forms an asymptote at certain concentration of the substrate, which is immediately followed by an insignificant change in the production even with the increase in substrate dosage-known as substrate inhibition.
ro of
Unavailability of substrate binding site for the active site of the enzyme manifests towards an insignificant change in production.
Like cellulose, enzymatic hydrolysis of hemicellulose is another important concern as the
-p
lignocellulosic biomass is primarily consisted of 25-30% (w/w) of hemicellulose. Xylan is the major backbone of hemicellulosic fraction of biomass and xylanase has been the prime
re
enzyme for hydrolyzing xylan for the production of xylose at favorable reaction conditions.
lP
Further, a synergistic working mechanism of endo-xylanase, exo-xylanase and β-xylosidase breaks the hemicellulosic network of any lignocellulosic biomass. Endo-xylanase acts internally on the main chain of xylan polymer to produce xylo-oligomers whereas exo-
na
xylanase cleaves externally on the reducing ends and finally β-xylosidase produces xylose from xylo-oligomers. Like xylanase arbinosidase produces arabinose after hydrolyzing
ur
arabinan polymer of lignocellulosic biomass.
Jo
3.1. Factors influence the process of enzymatic hydrolysis A number of different substrate and enzyme related factors influence the process of enzymatic hydrolysis. Among the substrate related factors cellulose crystallinity is most important in assessing the rate of enzyme enzymatic hydrolysis. It was observed that, process of pretreatment is applied to reduce cellulose crystallinity by decreasing the particle size of the biomass. However, some commercial cellulase mixture is observed to effectively
33
hydrolyze crystalline cellulose [115]. Apart from cellulose crystallinity, the availability of substrate binding site for the active site present in the enzyme influences the progress of enzyme hydrolysis. The purpose of pretreatment is aimed to increase the surface area available for enzymatic hydrolysis. Further, lignin stands as major barrier for hydrolysable substrate fraction towards enzymatic hydrolysis. Apart from, the non-productive binding of lignin on enzyme typically retards the ability of active site to bind with accessible region of substrate molecule [116]. A number of different strategies including addition of protein and
ro of
additives are investigated to overcome the problem associated with non-productive adsorption of lignin onto cellulase [117]. Finally, an increase in porosity and reduction in particle size significantly improves the process of enzymatic hydrolysis.
-p
3.2. Cellulase enzyme: Biochemistry and mechanistic interactions
Primarily, cellulase enzyme involved in the hydrolysis of biomass is mostly isolated from
re
Trichoderma sp. [108, 110]. In view of that, cellulolytic mixture particularly obtained from
lP
Trichoderma sp. is believed to act synergistically for an effective disintegration of polymeric carbohydrates. Exoglucanases, CBHI (Cellobiohydrolase-I) and CBHII (CellobiohydrolaseII) releases cellobiose molecules from the end of cellulosic chains and endoglucanases cut the
na
chain at inside of any places within the cellulose. While β-glucosidase finally relieves the glucose molecules out the produced cellobiose [118, 119]. Hence, an instantaneous
ur
breakdown of cellobiose into glucose molecules avoids the feasibility of product inhibition by
Jo
cellobiose molecules. Fig. 4 depicts the synergistic action of cellulase enzyme to produce glucose molecules from cellulose. Now, as per as progress of the hydrolytic reaction is concerned, the combined activity of cellulose binding domain (CBD) and catalytic domain (CD) have been one of the two most critical factors. CBD facilitates the binding of cellulase enzyme on the surface of cellulose containing biomass while CD activates the catalysis involved in the hydrolytic production of glucose molecules. Any conformational changes
34
within these domains lead to impair the progress of hydrolysis. With regard to an improper interactions between –OH group of sugars with –CONH2 group of CBHI leads towards conformational change within the active site of the exo-glucanse enzyme [120]. Cel7A has been the most termed exoglucanse as the CBHI accounts around 60% of total protein secreted by Trichoderma reesei. A number of different aromatic amino acids such as tyrosine, phenylalanine and non-polar amino acids like leucine, isoleucine was found distributed over the surface of the enzyme. Glu212 and Glu217 was observed as the two most active amino
ro of
acids which takes part in the formation of an intermediate enzyme-cellobiose complex mediated through nucleophilic reaction. Finally, subsequent protonation of cellobiose molecules manifested towards the production of glucose from the intermediate complex
-p
[121].
3.3. Efforts to overcome the criticalities involved with cellulase
re
A number of different factors involved with cellulase enzyme limits full fledge
lP
commercialization of enzymatic hydrolysis of lignocellulosic biomass. Among the others, high cost of enzyme along with the performance on substrate molecules is often considered as the most dictating factors of the overall process of enzymatic hydrolysis. Reduction in the
na
cost of the enzyme is in line with the requisite support to provide an optimum growth condition for the microbes towards the production of cellulase enzyme. In view of that, an
ur
introduction of submerged fermentation or solid-state fermentation using substrate specific
Jo
microbes are widely reported [122]. To reduce an overall cost of the process, enzyme immobilization is recognized as the most effective strategy to reuse the enzyme for repetitive times with almost similar activity [123]. Further, the problem of feedback inhibition due to an instant accumulation of cellobiose during the hydrolysis of cellulose. In order to overcome the accumulation of cellobiose, supplementation of β-glucosidase in cellulase is studied to limit the restriction on enzyme-cellulose complex caused by cellobiose molecules.
35
Moreover, apart from acting as a barrier for cellulolytic enzymes, lignin is observed to involve in non-productive binding with the amino-acids present in the active site of cellulase enzyme thus manifesting a reduction in enzymatic production of sugars. A number of different strategies including an addition of additives such as surfactants and proteins are observed to significantly improve the rate of enzymatic production through the binding with lignin molecules [117, 124].
4. Strategies involved in the fermentation of sugars into biofuel
ro of
Primarily, the hydrolysate obtained after chemical and enzymatic hydrolysis has been used as feedstock material for fermentation process. In view of that, a number of fermentation strategies studied for different lignocellulosic biomass using a horizon of microorganisms.
-p
Table 7 highlights on few of the top most companies around the globe working in the area of biofuel using low cost biomass. Among them, Abengoa bioenergy has recently made
re
significant breakthrough using a novel enzymatic process towards the production of 25
lP
million gallon of ethanol per year followed by an annual production of 42 million liters of bioethanol by Raizen-Iogen at the unit located in Brazil. Out of the several strategies,
na
separate hydrolysis fermentation (SHF) and simultaneous saccharification fermentation (SSF) are mostly used during the fermentation of hydrolysate obtained from lignocellulosic
ur
biomass. With regard to SHF, SSF is mostly preferred due to the ability of the system to overcome the problem of product inhibition [125]. Still, the difference in the optimum
Jo
reaction temperatures for hydrolysis and fermentation demands technological improvements in order to facilitate the progress during the process of fermentation. With regard to that, an incorporation of thermo-tolerant microbes seems to be adaptable even at high temperature required for enzymatic hydrolysis. Now as per as fermentation product is concerned, biobutanol has received much attention over bioethanol as the former contains high energy content, lower heat of vaporization, less corrosive, substantial intermixing property [126]. 36
Most of the recent research investigations on biobutanol production acknowledge ABE pathway
using
species
such
as
Clostridium
acetobutylicum,
C.
beijerinkii,
C.
saccharobutylicim [127, 128]. 4.1. Consolidated bioprocess (CBP) involved in fermentation The term consolidated arises as the entire production process of biomass to biofuel is accomplished into a single reactor. During this process, microbes produce enzyme, which leads to disintegrate the biomass itself for the production of biofuel within the reactor [129,
ro of
130]. Hence, on an economical perspective, the process of consolidated bioprocess has emerged as one of the promising approaches compared to the process in which enzyme production, saccharification and fermentation are carried out separately. Jiang et al. carried
-p
out CBP using a new thermos anaerobacterium sp M5 for the xylan as substrate and at 55 °C temperature, 1.1 g/L of butanol was produced which was enhanced to 8.2 g/L when a mix
re
culture of thermoanaerobacterium sp M5 and C. acetobutylicum was employed in CBP
lP
[131]. In an another approach, consolidated bioprocess was employed to produce bioethanol with a yield of around 67% from the slurry obtained from dilute acid treated wheat straw of 17 g/L of substrate concentration [132]. The effectiveness of consolidated bioprocessing
na
shows an isopropanol production of ~8.4 g/L when consortium of EMSD5 was explored for the conversion of arabinoxylan of corncob [133].
ur
4.2. Simultaneous saccharification and co fermentation
Jo
Simultaneous saccharification and fermentation is carried out with a set of microorganisms which possess the significant capability to ferment pentose and hexose sugars together at same reaction conditions. Liu et al. carried out simultaneous saccharification and co fermentation of S.cerevisiae and C. tropicalis at different ratios, temperature and pH. Under optimum reaction conditions of pH 5, temperature of 32 °C in 144 h of reaction period a maximum of 11 g/100 mL of bioethanol was obtained [134]. So, microorganisms are well
37
capable of an efficient utilization of mixture of pentose and hexose sugars present in biomass hydrolysate for the production of biofuels. However, few microbes are reported as incapable of utilizing the pentose sugars present in the feedstock material for fermentation [135]. Although E. coli exhibits an inherent property for a parallel utilization of both of the pentose and hexose sugars as substrate though, along with other engineered microorganisms E.coli was also observed to show the phenomenon of carbon catabolite repression (CCR). In view of that, CCR is known as metabolic regulation of any microorganism to exhibit their
ro of
preferences of using a particular sugar over others present in the hydrolysate sample, which ultimately attributes towards a reduction in the production of ethanol at the end. In order to mitigate such bottleneck of CCR, continuous culture facilitates towards a significant
-p
consumption of sugars and reduction in fermentation time. Fernandez-Sandoval et al. conducted single stage continuous cultures using micro-aerated conditions and as a result,
re
steady state ethanol production of 18 g/L was observed when glucose of 7.5 g/L and xylose
lP
of 42.5 g/L was consumed together as a mixture in mineral medium [136]. Although coconsumption of multiple sugars as carbon sources may be improved in microorganism after modulating the genes including mgsA, pgi in E. coli involved in phosphotransferase systems
na
(PTS) but due to such anticipation, ethanol production was negatively affected. On the contrary, modulation on the genes such as; galP, glk of non PTS pathways was studied to
ur
exhibit an improved production in cellulosic ethanol [137]. Further, inactivation of gid and
Jo
ccp genes directs carbon catabolite repression in Clostridium sp. MF28 with a simultaneous ability to co-ferment glucose, xylose and arabinose together for the production of around 12 g/L butanol [138]. 4.3. Simultaneous pretreatment and saccharification During biological pretreatment, few microorganisms have been observed to deconstruct the structure of lignocellulosic biomass and subsequent saccharification into monomeric sugars.
38
In this regard, an attempt to find the ligninolytic fungi with cellulolytic enzymes may be beneficial for improved saccharification of biomass. In this particular phenomenon of coexistence, different reaction conditions of delignification and enzymatic saccharification negatively impact the overall saccharification performance of the process and the separation of fungi has been another concern after the pretreatment process [139]. Karimi et al. studied simultaneous delignification and saccharification of rice straw by immobilized Trichoderma viride and under optimal reaction condition, lignin removal efficiency of around 74% and 8.5
ro of
g/L sugars were obtained after 10 days of pretreatment [140]. Moreover, a number of different microbes such as S. cerevisiae, Z. mobilis, P.stipitis are commonly used during the fermentation of biomass. Among different microbes, few microbes like S. cerevisiae is well
-p
known to ferment hexose sugars while Pichia sp is specialized for pentose sugars. An efficient conversion of biomass to biofuel demands involvement of microbes able to ferment
re
both of the pentose and hexose sugars. Hence, genetic modification in different
lP
microorganisms are immensely focused to improve the fermentation ability for both pentose and hexose sugars together in a single reaction system. Table 8 briefed on various fermentations strategies including merits and demerits of the individual methods.
na
4.4. Exploration of metabolic engineering towards improved biofuel production Commercial installation of fermentation technology has been impeded due to some existing
ur
bottleneck of limited yield and inappropriate substrate consumption by the microbes. In order
Jo
to rectify the concerns, understanding the metabolism or genetic modifications of the organism has been an important aspect. As per as large scale fermentation is concerned, S. cerevisiae and Z. mobilies are among mostly employed microorganisms for the lignocellulosic ethanol production [141-144].
39
4.4.1. Engineered saccharomyces cerevisiae for bioethanol production Apart from the ability of producing high yield of ethanol from a number of lignocellulosic biomass, S. Cerevisiae has also been observed to exhibit the properties of high tolerance against osmotic pressure and viral infections [145, 146]. The fungal strain has been studied with few limitations when substrate is xylose. On the other side, around 35 to 40% of the feedstock is consisted of xylose. So, in order to increase rapid xylose fermenting ability of the strain, glucose repression on xylose fermentation need to overcome. With regard to that, most
ro of
of the natural yeast strain has been observed to ferment xylose under anaerobic condition thus not amenable to industrial context. Hence, development of an industrial strain has always been a top research endeavor to overcome the existing issues. Though, most of the industrial
-p
strain has been observed as polyploidy thus attributes to perform sporulation and HO gene deletion to convert them into haploid or diploid which often losses the advantageous trait of
re
polyploids [147]. To get rid of such intricacies, recent development of Cas9 based genome
lP
editing technology has been developed to insert and delete target gene within the polyploid yeast strain [148]. Lee et al. introduced Cas9 based genome editing on S. cerevisiae strain to construct an auxotrophic engineered strain, JX123 after incorporating xylose metabolic
na
pathway. Additionally, the accumulation of xylitol was significantly reduced through NADH oxidase from L. lactis [141]. Moreover, simulation based study revealed that, glucose act as a
ur
potent inhibitor for S. cerevisiae during xylose uptake when substrate medium is consisted of
Jo
both of the sugars [149]. Despite of numerous attempt to advance genetic engineering, glucose transporters exhibited limited affinity to transport xylose through facilitated diffusion within the cell [150]. Hence, both are consumed simultaneously at glucose limited conditions. Lane et al. studied genome sequencing of a mutant stain and reported that, mutation in glucose phosphorylating enzymes such as hexokinase1, hexokinase2 and glucokinase1 manifested simultaneous uptake of glucose and xylose [151]. Likewise, Farwick et al.
40
reported that, mutations at asparagine and threonine amino acids located at transmembrane Gal2 and Hxt7 transporter substantially facilitate the transport of xylose without any inhibition caused by glucose [152]. 4.4.2. Engineered Zymomonas mobilis for bioethanol production Among different fungi, Zymomonas mobilis has been one of the well-recognized ethanologenic microorganism to efficiently convert glucose into bioethanol [153, 154]. Also, both the organisms S. cerevisiae and Z. mobilis uses the common platform of bioethanol
ro of
production. However, Z. mobilis uses Entner-Dioudoroff pathway instead of using EmbdenMeyerhof-Parnas pathway (used by S. cerevisiae) for ethanol production thereby consumed less ATP compared to S. cerevisiae. Further, on a specific amount of hexose sugar, Z. mobilis
-p
was studied to produce less biomass compared to other yeast cells. Thereby, based on the numerous advantages, Z. mobilis was considered as model organism for higher ethahol yield
re
[155]. Like S. cerevisiae, this organism is also observed with some limitation during the
lP
fermentation of pentose sugars specially xylose and arabinose. So, a number of different approaches were studied to overcome the fermentation ability of pentose sugars by Z. mobilis. Five different genes encoding araA, araB, araD, talB, tktA was isolated from E.coli
na
bacterium and introduced into Z. mobilis and more than 90% of theoretical ethanol yield was reported utilizing only arabinose as sole carbon source [156].
ur
4.4.3. Genetic engineering in Clostridium acetobutylicum for biobutanol production
Jo
Clostridium acetobutylicum is well established to produce butanol using glucose, xylose and arabinose obtained from lignocellulosic biomass. Primarily, the pentose sugar, xylose with an accompany of glucose constitute the major portions of the hydrolysate obtained from biomass. During the persistence of glucose repression, utilization of xylose mainly depends on xylose isomerase, xylulokinase and enzymes of pentose phosphate pathway [157]. To improve xylose uptake efficiency, gene knock out system was adopted to inactivate glcG
41
gene of D-glucose phosphoenolpyryvate dependent phosphotransferase system. Further, around 24% increase in butanol yield was observed when other relevant genes were coexpressed in the GlcG mutant strain. But, less improvement in xylose uptake still demands further investigations with the engineering of specific xylose transporters. In an another study, around 12 g/L of butanol was produced when the gene encoded the CcpA protein was discarded from C. acetobutylicum ATCC 824 manifested substantial improvement in simultaneous glucose and xylose uptake in the absence of catabolite repression [158].
ro of
5. Conversion of biomass into the derivatives of cellulose and lignin
Growing concern over fossil fuel depletion reflects much attention on sustainable conversion of lignocellulosic biomass into value-added products. Based on the fact, biomass conversion
-p
has been immensely acknowledged at everywhere in academics and industries for a valuable transformation of biomass into fuel and platform chemicals [24, 159]. Over the years,
re
lignocellulosic biomass has been the cheap, abundant and most viable source to gather
lP
cellulose, hemicellulose and lignin in its polymeric form. Among these, cellulose and hemicellulose represents around 70-75% of total biomass and most of the rest is consisted of
na
polymeric phenols such as lignin. Now conversion of cellulose into glucose, gluconic acids, HMF, lactic acid and levulinic acid has already investigated and received massive
ur
acknowledgement [160]. Nevertheless, promising approaches involving greener routes towards the production of sustainable fuel and chemicals from cellulose and lignin are briefed
Jo
over here.
5.1. Derivatives of cellulose Among the derivatives obtained from cellulose, different acids, alcohols and HMF are well known.
42
5.1.1. Formic acid Formic acid is well known as the simplest form of carboxylic acid primarily used as storage form of hydrogen as it can be efficiently converted into carbon-di-oxide and hydrogen through rapid decomposition of formic acid by catalytic hydrothermal reaction [161]. It was apparently seen that, yield of formic acid is much higher from glucose than cellulose [162]. Under protonated condition, cellulose is hydrolyzed into glucose and in presence of an oxidant, glucose is oxidized into formic acid at higher temperature and pressure. Hence,
ro of
oxidative hydrothermal reaction facilitates the formation of formic acid from cellulose or glucose. In the presence of vanadyl ion (VO2+), oxidative hydrothermal reaction of cellulose leads to produce around 65% (w/w) of formic acid from glucose at temperature of 160 °C
-p
[163]. In an another experiment, cotton was alkylated to disrupt glycosidic linkages and a post hydrothermal reaction for 6 h leads to produce a maximum of 21.9% (w/w) formic acid
re
from cellulosic fraction [164].
lP
5.1.2. Acetic acid
Acetic acid is the second simplest form of carboxylic acid and suitable to use as vinegar for household purposes. Similar as formic acid, under strong protonated condition, glucose
na
molecules, the hydrolyzed sugar of cellulose is oxidized into acetic acid. Most of the hydrolysate obtained from the pretreatment of lignocellulosic biomass contains a certain
ur
amount of acetic acid. During acidic hydrolysis, cellulose hydrothermally degraded into
Jo
levulinic acid which further degraded into acetic acid under no oxygen condition [165]. In a similar work by Jin et al., acetic acid was produced using two-step process in which under alkali condition, first lactic acid was produced from biomass by hydrothermolysis reaction and followed by the production of acetic acid in the presence of H2O2 [166]. Still, usage of H2O2 was cost effective with insignificant improvement in the process. Thereafter, Huo et al. replaced H2O2 with CuO and observed an average of 23.7% yield in acetic acid from
43
carbohydrates such as glucose, cellulose and starch [167]. During the interaction with lactic acids, Cu2+ forms coordinate bond with two O atoms of lactic acids thereby decreases electronegativity of hydroxyl group. Further, nucleophilic attack by OH- of alkali on H atom of hydroxyl group of lactic acid subsequently leads to form acetic acid through an intermediate production of acetaldehyde and formic acid (Fig. 5). 5.1.3. Levulinic acid An efficient utilization of lignocellulosic biomass as a renewable feedstock for the production
ro of
of value-added chemicals not only save the cost but also mitigate the problems of greenhouse gasses. Among a number of different chemicals, levulinic acid has received much attention for the preparation of pharma products, polymers, plastic, resins and last but not the least can
-p
be used as fuel with gasoline without the any modification in engine architecture [168]. A number of research investigation has reported that, levulinic acid is produced directly from
re
acidic hydrolysis of lignocellulosic biomass. Among the acids, sulphuric acid is mostly used
lP
at a range of 0.1 to 2 mol/L to catalyze hexose sugars like glucose and fructose. During acid hydrolysis, three molecules of water is removed from hexose sugars as a result of dehydration at a temperature of 140-220 °C followed by a rapid conversion into levulinic acid [71, 169].
na
Jeong et al. showed that, modified zeolite in the presence of 0.25 M NaOH produced 4.7% of levulinic acid from bamboo at 190 °C for 30 min [170]. In an another study, 47.5% yield of
Jo
[171].
ur
levulinic acid was observed when bamboo was treated with acidic ionic liquid at 100 °C
5.1.4. Sugar alcohol Sugar alcohols are of immense importance as it is being widely used in food industries and produced from cellulose and glucose of lignocellulosic biomass through hydrogenation reactions. In view of that, mostly produced sugar alcohols are mannitol and sorbitol catalytically obtained from glucose in the presence of hydrogen [172]. Further metal catalyst
44
such as Pt, Pd and Ru in combinations with sulphuric acids were observed towards a conversion of 60% into C4 and C6 sugar alcohols in an hour when the biomass was spruce [173]. Additionally, direct conversion of softwood (Conifers and gymnosperm trees produces softwood biomass and seeds of this plants are observed as uncovered) cellulose into 5C and 6C sugar alcohols using supported platinum catalyst in water was also studied in a hydrogenolysis reaction [174]. Yamaguchi et al. developed Ru and Pt based metal carbon catalyst which was observed to produce 55.1% (w/w) sugar alcohols directly from ball milled
ro of
hard wood biomass [175]. 5.1.5. Hydroxymethyl furfural (HMF)
Hydroxymethyl furfural (HMF) has been one of the top most valuable intermediate which is
-p
primarily obtained from a number of different lignocellulosic feedstocks. In an addition, HMF is also produced from synthetic monosaccharides, disaccharides and polysaccharides.
re
An application of mostly used catalyst such as minerals and organic acids facilitate towards
lP
an improved production of HMF. Besides, HMF is recognized as “sleeping giant” by some authors due to its innumerable importance to act as valuable intermediate for the formation of a number of platform chemicals. Nguyen et al. reported that, a maximum yield of around 79
na
mol% for HMF was obtained when biomass was first treated with 3% (w/w) NaOH and subsequently catalyzed in a reaction system comprising of 1-butyl-3-methyl imidazolium
ur
chloride ([BMIM]Cl) as ionic liquid and CrCl3.6H2O used as the catalyst [176]. On a
Jo
mechanistic aspect, HMF is mainly produced from acidic dehydration of hexose sugars primarily glucose. In the presence of strong acidic conditions, three molecules of water move out of the glucose molecule and subsequently transformed into HMF.
45
5.2. Derivatives of lignin The cleavage of the predominant C-O type linkages within lignin molecules leads to thermochemical degradation into number of chemicals subcategorized into phenols, aldehydes and acids. 5.2.1. Phenol Hydrogenolysis, acedolysis, depolymerization are among promising methods to convert lignin into value-added products. The effect of hydrogenolysis has been quite rare event for
ro of
lignin conversion and on the other hand a severe problem with lignin depolymerization has been immense as re-condensation of the lignin fragments leads to form non cleavable bonds in insoluble lignin. In order to overcome re condensation of lignin fragments, lignin was
-p
reacted with formaldehyde to form 1,3-dioxane structure thus the condensation reaction is prevented by forming a soluble lignin. Further, the presence of more C-O type bonds in the
re
soluble lignin facilitates the hydrogenolysis of lignin into guaiacyl and syringyl monomeric
lP
units [177]. Want et al. studied the effect of different metal formate on the kraft lignin conversion into polyphenols. Around 23.7% (w/w) guaiacol was obtained from pure lignin and the production of polyphenols like different catechols was observed to significantly
na
increased with the addition of metal formats. A maximum yield of 22% of catechol was observed in the presence of nickel formate [178]. Incorporation of bronsted and lewis acid
ur
into the reaction medium was investigated to cleave C-O bonds present in lignin and
Jo
subsequently leads to produce monomeric units under mild acidic condition [179]. However, all such methods in presence or absence of hydrogen still faces a lot of challenges including corrosion, metal recovery to successfully implemented on an industrial basis. Apart from, in the presence of high temperature in the range of 300-1000 °C, lignin undergoes deformation into pyrolytic aromatic monomers such as mixture of phenols [180]. Likewise, exploration of the supercritical conditions of methanol or ethanol to yield catechol is another promising
46
method of producing phenolic compounds when alkylated lignin was used [181]. A number of different strategies involved in the conversion process of lignin is depicted in fig. 6. Fig. 6 (A) shows disintegration of lignin, which resulted in the ring openings and subsequent oxidation. Fig. 6(B) and (C) shows openings of the rings and conversion into carboxylic acids such as fumaric, maleic, muconic acids, succinic, malic and malonic acid. 5.2.2. Vanillin On a nut shell, vanillin has been the major form of phenol aldehydes industrially obtained
ro of
specifically from guaiacyl and syringyl units of lignin of lignocellulosic biomass [182]. It has been reported that, lignin obtained from sulphite pulping is much expensive compared to vanillin from guaiacol units [183]. During the production of vanillin, nitrobenzene was used
-p
as one of the staring reagent for oxidative depolymerization reaction of lignin [184]. But, toxic effect of nitrobenzene and separation problem with aniline as by product substituted
re
oxygen as alterative oxidant. Around 15% (w/w) mixture of vanillin and syringaldehyde was
5.2.3. Carboxylic acids
lP
obtained from steam exploded lignin in the presence of copper or ferrous ions [185].
During oxidation of linin, carboxylic acids are also produced as a side products with aromatic
na
aldehydes and phenols [186]. As lignin is the polymerized form of p-hydroxyphenyl, guaiacyl and syringyl subunits so, oxidation of lignin attibutes towards wide variety of carboxylic
ur
acids including aliphatic and aromatic with one or more carboxyl groups [187]. As these
Jo
acids are produced from fossil fuel based products so exploration of the renewable lignin has been a sustainable approach towards greener production of carboxylic acid. Now a days, research work is more focused on the conversion of aliphatic acid compared to aromatic acids as the market value of the aliphatic acids such as maleic acid and succinic acids are relatively high. Oxidation of lignin has been so effective process to disrupt the closed aromatic structure towards the subsequent formation of acids. In view of that, two different lignins
47
obtained from acid pretreated and steam pretreated biomass was oxidized using copper ferrous sulphide and in the presence of hydrogen peroxide, an average of 14% and 11% of dicarboxylic aliphatic acids are produced respectively, from two different lignins [188]. In the recent years the generation of hydroxyl radicals using Fenton’s reagent with H2O2 has been emerged as an effective oxidative media for lignin conversion into different aromatic acids [186, 189]. 5.3. Detoxification of inhibitors
ro of
As per as industrial acceptability is concerned, detoxification of fermentation inhibitors (originated from the conversion process of carbohydrate and lignin fraction of biomass) is of high importance. An effective detoxification method facilitates substantial elimination of
-p
inhibitors from the hydrolysate and reduce the impact of negative interference during the subsequent process of fermentation. Furfural generated from pentose sugars and
re
hydroxymethyl furfural obtained from hexose sugars of the biomass are detoxified after an
lP
evaporation of the hydrolysate with the aid of physical detoxification strategy [190]. In view of that, detoxification using activated carbon is an old and routine practice for an efficient removal of inhibitors without interfering the sugars present in the hydrolysate as mixture. In
na
order to prepare an inhibitor free feedstock for the fermentation obtained after acid hydrolysis of biomass, an addition of sodium hydroxide and calcium hydroxide into the reaction mixture
ur
influence the neutralization reaction for formic acid, acetic acid and levulinic acid present in
Jo
the hydrolysate [191]. Jeong et al. studied the effectiveness of complex extractant comprising a mixture of 20% trialkylamine, 70% of n-octanol and 10% of kerosene, which manifests towards 64% removal of furfural, acetic acid, formic acid and levulinic acid from the hydrolysate mixture [192]. Further, the presence of 1 g/L of lignin derivatives including – coumaric acid, ferulic acid, vanillin, syringaldehyde, 4-hydroxybenzoic acid inhibited more than 70% of cell growth in C. beijerinckii and completely paused butanol production while an
48
addition of 0.01 µmole of peroxidases enzyme significantly improved the cell growth and butanol production after detoxification of phenolic inhibitors [193]. Moreover, Zhang et al. observed 5% (w/v) of activated carbon effectively eliminated 78% of furan derivatives and 98.6% of aromatic monomers as inhibitors present in the poplar pre-hydrolyzate prior to the fermentation into biobutanol production.
6. Future perspective The entire aspect of lignocellulosic conversion is described with an effort to establish the
ro of
concept of valorization using biomass residues into value-added products. In view of that, a series of attempts were made to enumerate the effective processes based on their economic viability and demerits that can be used in tandem to produce value-added products from
-p
lignocellulosic feedstock. Hence, a number of different mechanical, physicochemical, chemical and biological pretreatment methods are comprehensively narrated as one of the
re
most critical and cost determining unit operations involved for an effective removal of the
lP
lignin component of the biomass. Primarily, mechanical operations, including ball milling and extrusion are observed with an ability to deconstruct the intricate structure of biomass at
na
an expense of mechanical and electrical energy. Whereas, physicochemical treatments like hot water, steam explosion and wet oxidation accounted for the rapid disruption in
ur
lignocellulosic structure with an involvement of high-pressure reactor. Further, chemical pretreatments are investigated to solubilize certain fraction of lignocellulosic components
Jo
when biomass is subjected to chemically treated with acid or base at certain reaction conditions. Acid hydrolysis is observed to solubilize maximum portion of hemicellulose and hydrolyze part of the cellulose of biomass in the prehydrolyzate along with the presence of few unwanted by-products known as inhibitors for fermentation process. As per as economic feasibility of the process is concerned, acid hydrolysis is hardly considered for large scale of industrial adaptation as the process of detoxification mandates significant elimination of the 49
toxic inhibitors prior to the fermentation process attributes towards an additional expense at the downstream processing. On the contrary, alkaline treatment using sodium hydroxide is considered as the most effective chemical pretreatment method as the process involves substantial delignification at an expense of mild reaction conditions. Based on the ground reality, other chemical treatments including ionic liquid and organic solvents necessitates the additional steps of recovery and reuse of the useful streams when economic acceptability of the process is concerned. Over the years, a number of various attempts are made in biological
ro of
pretreatment for an effective fractionation of biomass components with respect to time and process condition. However, the process of biological pretreatment demands subsequent opportunity to develop a rapid and cost-effective method for lignocellulosic biofuel
-p
production. On very brief note to say, the sole efficiency of any pretreatment process depends on the selection of the biomass based on the composition of cellulose, hemicellulose and
re
lignin. Hot water pretreatment may be an effective approach for the biomass with low lignin
lP
content as the process does not require the extraneous addition of any chemicals. On the contrary, the biomass with high content of lignin demands an efficient delignification process using mild alkaline reaction condition prior to enzymatic hydrolysis. The process of
na
enzymatic hydrolysis is mostly preferred as no co-formation of inhibitors are produced with monomeric sugars at the product side. Though enzymatic hydrolysis is considered as the most
ur
costly affair in the conversion process of biomass, but an application of integrated enzymes
Jo
from different sources attributes towards an increase in the production. However, the process cost invariably increases with the variation in enzyme application. Instead, the development of an enzyme with combined activity of cellulase, hemicellulase and ligninase encourages the feasibility of integrating the process of pretreatment and hydrolysis into a single cost effective step. During an initial phase of enzymatic hydrolysis, a rapid production of sugars is followed by a sudden declination in the sugars formation, which manifests towards the aspect
50
of product inhibition that interfere with the action of enzyme and process of hydrolysis. Following hydrolysis, another attempt was made to explore the hydrolytic and fermentation efficiency of lignocellulosic biomass for the production of bioethanol and biobutanol as biofuel. As per as fermentation is concerned, two subsequent processes of enzymatic hydrolysis and fermentation invariably increase the cost factor and make the process economically unsuitable. On the contrary, subsequent processes of hydrolysis and fermentation are simultaneously carried out together into a single reactor. However, there are
ro of
differences in the optimum reaction conditions of enzymatic hydrolysis and fermentation process. Therefore, both of the conventional modes of fermentation processes urge an indepth investigation by aiming at maximum yield using pretreated biomass as the feedstock
-p
material for biofuel production. Finally, various routes of producing the derivatives of cellulose and lignin into the valuable products directs the opportunity of exploring
re
lignocellulosic biomass as an indispensable option for biomass valorization.
lP
7. Conclusions
Apart from being a sustainable alternative, biofuel not only reduces the effect of greenhouse
na
gas emission, it also eases the much dependency on petroleum based conventional fuels. Like other global departments, The Government of India has already mandated a certain
ur
percentage of ethanol blending as a part of biofuel programme. Nevertheless, full-fledged commercialization of biofuel production still demands substantial improvements in the
Jo
existing technologies. Therefore, advancement in different pretreatment operations enable to decide on the selection of most cost economic option. Therefore, an exhaustive review on the readily available pretreatment methods along with the emerging technologies assist to build an overview of the initial unit operation. In view of that, pretreatment of biomass using hot water may be a promising approach as the process doesn’t demand any extraneous addition of chemicals to disrupt the structural intricacy of lignocellulosic residues. In the consequent 51
process, a detail understanding involved with the mechanistic interaction during enzymatic hydrolysis facilitates the production of fermentable sugars from pretreated biomass. With regard to the enzymatic system, hydrolysis of biomass in the presence of cellulase along with accessory enzymes excels the breakdown of β-(1-4) glycosidic bonds of polymeric carbohydrates for an effective saccharification process. However, as per as enzyme reusability is concerned, an efficiency of enzymatic process significantly depends on the cost of the enzyme. Further, an exploration of genetically modified microbes that primarily
ro of
possess the properties of cellulase, hemicellulase and cellobiase would have been most effective fermentation strategy during consolidated bioprocessing. Finally, an in depth knowledge of various biochemical routes involved in the transformation of cellulose and
-p
lignin into their derivatives critically analyzed the entire aspect of lignocellulosic conversion
re
process of biomass to biofuel to the readers.
lP
Disclosure statement
The authors declare that, there is no conflict of interest with the work for the present review
na
manuscript.
ur
Conflict of interest
The authors declare that, there is no conflict of interest with the work for the present review
Jo
manuscript (Ref No. PRBI_2019_908). Authors: Dibyajyoti Haldar, Mihir Kumar Purkait
52
Acknowledgement Centre for the Environment and R&D (Research and Development) section of IIT Guwahati is sincerely acknowledged for providing infrastructural facilities and institute funds through
Jo
ur
na
lP
re
-p
ro of
MHRD (Ministry of Human Resource and Development), Government of India.
53
References
Jo
ur
na
lP
re
-p
ro of
[1] D. Carrillo-Nieves, M.J. Rostro Alanís, R. de la Cruz Quiroz, H.A. Ruiz, H.M.N. Iqbal, R. Parra-Saldívar, Current status and future trends of bioethanol production from agro-industrial wastes in Mexico, Renewable Sustainable Energy Rev. 102 (2019) 63-74. [2] E. Hosseini Koupaie, S. Dahadha, A.A. Bazyar Lakeh, A. Azizi, E. Elbeshbishy, Enzymatic pretreatment of lignocellulosic biomass for enhanced biomethane production-A review, J. Environ. Manage. 233 (2019) 774-784. [3] H.M. Zabed, S. Akter, J. Yun, G. Zhang, F.N. Awad, X. Qi, J.N. Sahu, Recent advances in biological pretreatment of microalgae and lignocellulosic biomass for biofuel production, Renewable Sustainable Energy Rev. 105 (2019) 105-128. [4] D. Haldar, D. Sen, K. Gayen, A review on the production of fermentable sugars from lignocellulosic biomass through conventional and enzymatic route—a comparison, Int. J. Green Energy 13(12) (2016) 1232-1253. [5] K. Pandiyan, A. Singh, S. Singh, A.K. Saxena, L. Nain, Technological interventions for utilization of crop residues and weedy biomass for second generation bio-ethanol production, Renewable Energy 132 (2019) 723-741. [6] V.K. Ponnusamy, D.D. Nguyen, J. Dharmaraja, S. Shobana, J.R. Banu, R.G. Saratale, S.W. Chang, G. Kumar, A review on lignin structure, pretreatments, fermentation reactions and biorefinery potential, Bioresour. Technol. 271 (2019) 462-472. [7] S.Y. Balaman, Introduction to Biomass—Resources, Production, Harvesting, Collection, and Storage, in: S.Y. Balaman (Ed.), Decision-Making for Biomass-Based Production Chains, Academic Press2019, pp. 1-23. [8] S. Mohapatra, R.C. Ray, S. Ramachandran, Bioethanol from biorenewable feedstocks: Technology, economics, and challenges, in: R.C. Ray, S. Ramachandran (Eds.), Bioethanol Production from Food Crops, Academic Press2019, pp. 3-27. [9] A. Avanthi, S. Kumar, K.C. Sherpa, R. Banerjee, Bioconversion of hemicelluloses of lignocellulosic biomass to ethanol: An attempt to utilize pentose sugars, Biofuels 8(4) (2017) 431-444. [10] S.B. Jamaldheen, K. Sharma, A. Rani, V.S. Moholkar, A. Goyal, Comparative analysis of pretreatment methods on sorghum (Sorghum durra) stalk agrowaste for holocellulose content, Prep. Biochem. Biotechnol. 48(6) (2018) 457-464. [11] C. Nitsos, L. Matsakas, K. Triantafyllidis, U. Rova, P. Christakopoulos, Investigation of different pretreatment methods of mediterranean-type ecosystem agricultural residues: Characterisation of pretreatment products, high-solids enzymatic hydrolysis and bioethanol production, Biofuels 9(5) (2018) 545-558. [12] Hemansi, R. Gupta, G. Yadav, G. Kumar, A. Yadav, J.K. Saini, R.C. Kuhad, Second Generation bioethanol production: The state of art, in: N. Srivastava, M. Srivastava, P.K. Mishra, S.N. Upadhyay, P.W. Ramteke, V.K. Gupta (Eds.), Sustainable Approaches for Biofuels Production Technologies: From Current Status to Practical Implementation, Springer International Publishing, Cham, 2019, pp. 121-146. [13] J.K. Saini, R. Gupta, Hemansi, A. Verma, P. Gaur, R. Saini, R. Shukla, R.C. Kuhad, Integrated lignocellulosic biorefinery for sustainable bio-based economy, in: N. Srivastava, M. Srivastava, P.K. Mishra, S.N. Upadhyay, P.W. Ramteke, V.K. Gupta (Eds.), Sustainable Approaches for Biofuels Production Technologies: From Current Status to Practical Implementation, Springer International Publishing, Cham, 2019, pp. 25-46. [14] D. Kumari, R. Singh, Pretreatment of lignocellulosic wastes for biofuel production: A critical review, Renewable Sustainable Energy Rev. 90 (2018) 877-891. [15] S. Chen, X. Zhang, D. Singh, H. Yu, X. Yang, Biological pretreatment of lignocellulosics: Potential, progress and challenges, Biofuels 1(1) (2010) 177-199. 54
Jo
ur
na
lP
re
-p
ro of
[16] M. Mohan, N.N. Deshavath, T. Banerjee, V.V. Goud, V.V. Dasu, Ionic liquid and sulfuric acid-based pretreatment of bamboo: Biomass delignification and enzymatic hydrolysis for the production of reducing sugars, Ind. Eng. Chem. Res. 57(31) (2018) 1010510117. [17] N.N. Deshavath, S.K. Sahoo, M.M. Panda, S. Mahanta, D.S.N. Goutham, V.V. Goud, V.V. Dasu, A. Jetty, The cost-effective stirred tank reactor for cellulase production from alkaline-pretreated agriculture waste biomass, Springer Singapore, Singapore, 2018, pp. 2535. [18] A. Althuri, A.D. Chintagunta, K.C. Sherpa, R. Banerjee, Simultaneous saccharification and fermentation of lignocellulosic biomass, in: S. Kumar, R.K. Sani (Eds.), Biorefining of Biomass to Biofuels: Opportunities and Perception, Springer International Publishing, Cham, 2018, pp. 265-285. [19] I. Ntaikou, N. Menis, M. Alexandropoulou, G. Antonopoulou, G. Lyberatos, Valorization of kitchen biowaste for ethanol production via simultaneous saccharification and fermentation using co-cultures of the yeasts Saccharomyces cerevisiae and Pichia stipitis, Bioresour. Technol. 263 (2018) 75-83. [20] V. Soti, S. Lenaerts, I. Cornet, Of enzyme use in cost-effective high solid simultaneous saccharification and fermentation processes, J. Biotechnol. 270 (2018) 70-76. [21] D. Chen, A. Gao, K. Cen, J. Zhang, X. Cao, Z. Ma, Investigation of biomass torrefaction based on three major components: Hemicellulose, cellulose, and lignin, Energy Convers. Manage. 169 (2018) 228-237. [22] J.Y. Yeo, B.L.F. Chin, J.K. Tan, Y.S. Loh, Comparative studies on the pyrolysis of cellulose, hemicellulose, and lignin based on combined kinetics, J. Energy Inst. 92(1) (2019) 27-37. [23] H. Yu, Z. Wu, G. Chen, Catalytic gasification characteristics of cellulose, hemicellulose and lignin, Renewable Energy 121 (2018) 559-567. [24] X. Han, Y. Guo, X. Liu, Q. Xia, Y. Wang, Catalytic conversion of lignocellulosic biomass into hydrocarbons: A mini review, Catal. Today 319 (2019) 2-13. [25] X. Li, R. Xu, J. Yang, S. Nie, D. Liu, Y. Liu, C. Si, Production of 5hydroxymethylfurfural and levulinic acid from lignocellulosic biomass and catalytic upgradation, Ind. Crops Prod. 130 (2019) 184-197. [26] H. Wang, C. Zhu, D. Li, Q. Liu, J. Tan, C. Wang, C. Cai, L. Ma, Recent advances in catalytic conversion of biomass to 5-hydroxymethylfurfural and 2, 5-dimethylfuran, Renewable Sustainable Energy Rev. 103 (2019) 227-247. [27] N. Sarkar, S.K. Ghosh, S. Bannerjee, K. Aikat, Bioethanol production from agricultural wastes: An overview, Renewable Energy 37(1) (2012) 19-27. [28] H. Chen, J. Liu, X. Chang, D. Chen, Y. Xue, P. Liu, H. Lin, S. Han, A review on the pretreatment of lignocellulose for high-value chemicals, Fuel Processing Technology 160 (2017) 196-206. [29] P. Alvira, E. Tomás-Pejó, M. Ballesteros, M.J. Negro, Pretreatment technologies for an efficient bioethanol production process based on enzymatic hydrolysis: A review, Bioresource Technology 101(13) (2010) 4851-4861. [30] L. Yang, F. Xu, X. Ge, Y. Li, Challenges and strategies for solid-state anaerobic digestion of lignocellulosic biomass, Renewable and Sustainable Energy Reviews 44 (2015) 824-834. [31] M. Kumar, K. Gayen, Developments in biobutanol production: New insights, Applied Energy 88(6) (2011) 1999-2012. [32] A. Arevalo-Gallegos, Z. Ahmad, M. Asgher, R. Parra-Saldivar, H.M.N. Iqbal, Lignocellulose: A sustainable material to produce value-added products with a zero waste approach—A review, International Journal of Biological Macromolecules 99 (2017) 308-318. 55
Jo
ur
na
lP
re
-p
ro of
[33] M. Bilal, M. Asgher, H.M.N. Iqbal, H. Hu, X. Zhang, Biotransformation of lignocellulosic materials into value-added products—A review, International Journal of Biological Macromolecules 98 (2017) 447-458. [34] M. FitzPatrick, P. Champagne, M.F. Cunningham, R.A. Whitney, A biorefinery processing perspective: Treatment of lignocellulosic materials for the production of valueadded products, Bioresource Technology 101(23) (2010) 8915-8922. [35] S. Poundrik, National Policy on Biofuels, Ministry of Petroleum and Natural Gas Notification, New Delhi, 2018, pp. 13-23. [36] M. Raud, T. Kikas, O. Sippula, N.J. Shurpali, Potentials and challenges in lignocellulosic biofuel production technology, Renewable and Sustainable Energy Reviews 111 (2019) 44-56. [37] P. Phitsuwan, K. Sakka, K. Ratanakhanokchai, Improvement of lignocellulosic biomass in planta: A review of feedstocks, biomass recalcitrance, and strategic manipulation of ideal plants designed for ethanol production and processability, Biomass and Bioenergy 58 (2013) 390-405. [38] W. Tyner, The US ethanol and biofuels boom: Its origins, current status, and future prospects, BioScience 58 (2008) 646-653. [39] W. Tyner, Comparison of the US and EU approaches to stimulating biofuels, Biofuels 1 (2010) 19-21. [40] H. Chen, M.-l. Xu, Q. Guo, L. Yang, Y. Ma, A review on present situation and development of biofuels in China, Journal of the Energy Institute 89(2) (2016) 248-255. [41] S.D. Musa, T. Zhonghua, A.O. Ibrahim, M. Habib, China's energy status: A critical look at fossils and renewable options, Renewable and Sustainable Energy Reviews 81 (2018) 2281-2290. [42] L. Kratky, T. Jirout, Biomass size reduction machines for enhancing biogas production, Chem. Eng. Technol. 34(3) (2011) 391-399. [43] G.G. Silva, M. Couturier, J.G. Berrin, A. Buleon, X. Rouau, Effects of grinding processes on enzymatic degradation of wheat straw, Bioresour Technol 103(1) (2012) 192200. [44] A. Barakat, C. Mayer-Laigle, A. Solhy, R.A.D. Arancon, H. de Vries, R. Luque, Mechanical pretreatments of lignocellulosic biomass: Towards facile and environmentally sound technologies for biofuels production, RSC Adv. 4(89) (2014) 48109-48127. [45] M. Lu, J. Li, L. Han, W. Xiao, An aggregated understanding of cellulase adsorption and hydrolysis for ball-milled cellulose, Bioresour Technol 273 (2019) 1-7. [46] Y.M. Gu, H. Kim, B.-I. Sang, J.H. Lee, Effects of water content on ball milling pretreatment and the enzymatic digestibility of corn stover, Water-Energy Nexus 1(1) (2018) 61-65. [47] Z. Yuan, J. Long, T. Wang, R. Shu, Q. Zhang, L. Ma, Process intensification effect of ball milling on the hydrothermal pretreatment for corn straw enzymolysis, Energy Convers. Manage. 101 (2015) 481-488. [48] S. Senturk-Ozer, H. Gevgilili, D.M. Kalyon, Biomass pretreatment strategies via control of rheological behavior of biomass suspensions and reactive twin screw extrusion processing, Bioresour. Technol. 102(19) (2011) 9068-9075. [49] A.S.A. da Silva, R.S.S. Teixeira, T. Endo, E.P.S. Bon, S.-H. Lee, Continuous pretreatment of sugarcane bagasse at high loading in an ionic liquid using a twin-screw extruder, Green Chem. 15(7) (2013) 1991-2001. [50] B. Lamsal, J. Yoo, K. Brijwani, S. Alavi, Extrusion as a thermo-mechanical pretreatment for lignocellulosic ethanol, Biomass Bioenergy 34(12) (2010) 1703-1710.
56
Jo
ur
na
lP
re
-p
ro of
[51] C. Karunanithy, K. Muthukumarappan, Effect of extruder parameters and moisture content of switchgrass, prairie cord grass on sugar recovery from enzymatic hydrolysis, Applied biochemistry and biotechnology 162(6) (2010) 1785-803. [52] M. Han, K.E. Kang, Y. Kim, G.-W. Choi, High efficiency bioethanol production from barley straw using a continuous pretreatment reactor, Process Biochem. 48(3) (2013) 488495. [53] Z. Lin, H. Huang, H. Zhang, L. Zhang, L. Yan, J. Chen, Ball milling pretreatment of corn stover for enhancing the efficiency of enzymatic hydrolysis, Appl. Biochem. Biotechnol. 162(7) (2010) 1872-1880. [54] Y. Sewsynker-Sukai, E.B. Gueguim Kana, Microwave-assisted alkalic salt pretreatment of corn cob wastes: Process optimization for improved sugar recovery, Ind. Crops Prod. 125 (2018) 284-292. [55] P.B. Subhedar, P. Ray, P.R. Gogate, Intensification of delignification and subsequent hydrolysis for the fermentable sugar production from lignocellulosic biomass using ultrasonic irradiation, Ultrason. Sonochem. 40 (2018) 140-150. [56] S. Tsubaki, J.-i. Azuma, S. Fujii, R. Singh, B. Thallada, Y. Wada, Microwave-driven biorefinery for utilization of food and agricultural waste biomass, in: T. Bhaskar, A. Pandey, S.V. Mohan, D.-J. Lee, S.K. Khanal (Eds.), Waste Biorefinery, Elsevier2018, pp. 393-408. [57] B.-M. Lee, J.-P. Jeun, P.-H. Kang, Enhanced enzymatic hydrolysis of kenaf core using irradiation and dilute acid, Radiat. Phys. Chem. 130 (2017) 216-220. [58] Y. Yin, J. Wang, Enhancement of enzymatic hydrolysis of wheat straw by gamma irradiation–alkaline pretreatment, Radiat. Phys. Chem. 123 (2016) 63-67. [59] K. Kapoor, N. Garg, R.K. Diwan, L. Varshney, A.K. Tyagi, Study the effect of gamma radiation pretreatment of sugarcane bagasse on its physcio-chemical morphological and structural properties, Radiat. Phys. Chem. 141 (2017) 190-195. [60] R. Liu, W. Tian, S. Kong, Y. Meng, H. Wang, J. Zhang, Effects of inorganic and organic acid pretreatments on the hydrothermal liquefaction of municipal secondary sludge, Energy Convers. Manage. 174 (2018) 661-667. [61] A. Mishra, S. Ghosh, Bioethanol production from various lignocellulosic feedstocks by a novel “fractional hydrolysis” technique with different inorganic acids and co-culture fermentation, Fuel 236 (2019) 544-553. [62] C.K. Nitsos, T. Choli-Papadopoulou, K.A. Matis, K.S. Triantafyllidis, Optimization of hydrothermal pretreatment of hardwood and softwood lignocellulosic residues for selective hemicellulose recovery and improved cellulose enzymatic hydrolysis, ACS Sustainable Chem. Eng. 4(9) (2016) 4529-4544. [63] Z. Yu, Y. Du, X. Shang, Y. Zheng, J. Zhou, Enhancing fermentable sugar yield from cassava residue using a two-step dilute ultra-low acid pretreatment process, Ind. Crops Prod. 124 (2018) 555-562. [64] Q. Liu, W. Li, Q. Ma, S. An, M. Li, H. Jameel, H.M. Chang, Pretreatment of corn stover for sugar production using a two-stage dilute acid followed by wet-milling pretreatment process, Bioresour Technol 211 (2016) 435-42. [65] S. An, W. Li, Q. Liu, Y. Xia, T. Zhang, F. Huang, Q. Lin, L. Chen, Combined dilute hydrochloric acid and alkaline wet oxidation pretreatment to improve sugar recovery of corn stover, Bioresour. Technol. 271 (2019) 283-288. [66] S. Chen, Z. Ling, X. Zhang, Y.S. Kim, F. Xu, Towards a multi-scale understanding of dilute hydrochloric acid and mild 1-ethyl-3-methylimidazolium acetate pretreatment for improving enzymatic hydrolysis of poplar wood, Ind. Crops Prod. 114 (2018) 123-131. [67] K. Rattanaporn, S. Roddecha, M. Sriariyanun, K. Cheenkachorn, Improving saccharification of oil palm shell by acetic acid pretreatment for biofuel production, Energy Procedia 141 (2017) 146-149. 57
Jo
ur
na
lP
re
-p
ro of
[68] P. Wang, Y.M. Chen, Y. Wang, Y.Y. Lee, W. Zong, S. Taylor, T. McDonald, Y. Wang, Towards comprehensive lignocellulosic biomass utilization for bioenergy production: Efficient biobutanol production from acetic acid pretreated switchgrass with Clostridium saccharoperbutylacetonicum N1-4, Appl. Energy 236 (2019) 551-559. [69] O.-M. Kwon, D.-H. Kim, S.-K. Kim, G.-T. Jeong, Production of sugars from macroalgae Gracilaria verrucosa using combined process of citric acid-catalyzed pretreatment and enzymatic hydrolysis, Algal Research 13 (2016) 293-297. [70] S.-Y. Jeong, J.-W. Lee, Optimization of pretreatment condition for ethanol production from oxalic acid pretreated biomass by response surface methodology, Ind. Crops Prod. 79 (2016) 1-6. [71] D. Haldar, D. Sen, K. Gayen, Development of spectrophotometric method for the analysis of multi-component carbohydrate mixture of different moieties, Appl. Biochem. Biotechnol. 181(4) (2017) 1416-1434. [72] H. Yang, Z. Shi, G. Xu, Y. Qin, J. Deng, J. Yang, Bioethanol production from bamboo with alkali-catalyzed liquid hot water pretreatment, Bioresour. Technol. 274 (2019) 261-266. [73] H. Li, X. Chen, L. Xiong, M. Luo, X. Chen, C. Wang, C. Huang, X. Chen, Stepwise enzymatic hydrolysis of alkaline oxidation treated sugarcane bagasse for the co-production of functional xylo-oligosaccharides and fermentable sugars, Bioresour. Technol. 275 (2019) 345-351. [74] M. Lainez, H.A. Ruiz, A.A. Castro-Luna, S. Martínez-Hernández, Release of simple sugars from lignocellulosic biomass of Agave salmiana leaves subject to sequential pretreatment and enzymatic saccharification, Biomass Bioenergy 118 (2018) 133-140. [75] K. Shill, S. Padmanabhan, Q. Xin, J.M. Prausnitz, D.S. Clark, H.W. Blanch, Ionic liquid pretreatment of cellulosic biomass: Enzymatic hydrolysis and ionic liquid recycle, Biotechnol. Bioeng. 108(3) (2011) 511-20. [76] Y. Fukaya, A. Sugimoto, H. Ohno, Superior solubility of polysaccharides in low viscosity, polar, and halogen-free 1,3-dialkylimidazolium formates, Biomacromolecules 7(12) (2006) 3295-7. [77] E.S. Sashina, D.A. Kashirskii, M. Zaborski, S. Jankowski, Synthesis and dissolving power of 1-Alkyl-3-methylpyridinium-based ionic liquids, Russ. J. Gen. Chem. 82(12) (2012) 1994-1998. [78] S. Saher, H. Saleem, A.M. Asim, M. Uroos, N. Muhammad, Pyridinium based ionic liquid: A pretreatment solvent and reaction medium for catalytic conversion of cellulose to total reducing sugars (TRS), J. Mol. Liq. 272 (2018) 330-336. [79] L.T.P. Trinh, Y.-J. Lee, C.S. Park, H.-J. Bae, Aqueous acidified ionic liquid pretreatment for bioethanol production and concentration of produced ethanol by pervaporation, J. Ind. Eng. Chem. 69 (2019) 57-65. [80] R.d.C.M. Miranda, J.V. Neta, L.F. Romanholo Ferreira, W.A. Gomes, C.S. do Nascimento, E. de B. Gomes, S. Mattedi, C.M.F. Soares, Á.S. Lima, Pineapple crown delignification using low-cost ionic liquid based on ethanolamine and organic acids, Carbohydr. Polym. 206 (2019) 302-308. [81] M.R. Sturgeon, S. Kim, K. Lawrence, R.S. Paton, S.C. Chmely, M. Nimlos, T.D. Foust, G.T. Beckham, A mechanistic investigation of acid-catalyzed cleavage of aryl-ether linkages: Implications for lignin depolymerization in acidic environments, ACS Sustainable Chem. Eng. 2(3) (2014) 472-485. [82] A. Satlewal, R. Agrawal, S. Bhagia, J. Sangoro, A.J. Ragauskas, Natural deep eutectic solvents for lignocellulosic biomass pretreatment: Recent developments, challenges and novel opportunities, Biotechnol. Adv. 36(8) (2018) 2032-2050. [83] E.K. New, T.Y. Wu, C.B. Tien Loong Lee, Z.Y. Poon, Y.-L. Loow, L.Y. Wei Foo, A. Procentese, L.F. Siow, W.H. Teoh, N.N. Nik Daud, J.M. Jahim, A.W. Mohammad, Potential 58
Jo
ur
na
lP
re
-p
ro of
use of pure and diluted choline chloride-based deep eutectic solvent in delignification of oil palm fronds, Process Saf. Environ. Prot. 123 (2019) 190-198. [84] Q. Yu, L. Qin, Y. Liu, Y. Sun, H. Xu, Z. Wang, Z. Yuan, In situ deep eutectic solvent pretreatment to improve lignin removal from garden wastes and enhance production of biomethane and microbial lipids, Bioresour. Technol. 271 (2019) 210-217. [85] Z. Chen, W.A. Jacoby, C. Wan, Ternary deep eutectic solvents for effective biomass deconstruction at high solids and low enzyme loadings, Bioresour. Technol. (2019). [86] A. Duque, P. Manzanares, I. Ballesteros, M. Ballesteros, Steam explosion as lignocellulosic biomass pretreatment, in: S.I. Mussatto (Ed.), Biomass Fractionation Technologies for a Lignocellulosic Feedstock Based Biorefinery, Elsevier, Amsterdam, 2016, pp. 349-368. [87] F.M.V. Oliveira, I.O. Pinheiro, A.M. Souto-Maior, C. Martin, A.R. Gonçalves, G.J.M. Rocha, Industrial-scale steam explosion pretreatment of sugarcane straw for enzymatic hydrolysis of cellulose for production of second generation ethanol and value-added products, Bioresour. Technol. 130 (2013) 168-173. [88] A. Verardi, A. Blasi, T. Marino, A. Molino, V. Calabrò, Effect of steam-pretreatment combined with hydrogen peroxide on lignocellulosic agricultural wastes for bioethanol production: Analysis of derived sugars and other by-products, J Energy Chem. 27(2) (2018) 535-543. [89] R. Kataria, A. Mol, E. Schulten, A. Happel, S.I. Mussatto, Bench scale steam explosion pretreatment of acid impregnated elephant grass biomass and its impacts on biomass composition, structure and hydrolysis, Ind. Crops Prod. 106 (2017) 48-58. [90] Z. Wang, G. Wu, L.J. Jönsson, Effects of impregnation of softwood with sulfuric acid and sulfur dioxide on chemical and physical characteristics, enzymatic digestibility, and fermentability, Bioresour. Technol. 247 (2018) 200-208. [91] G. Batista, R.B.A. Souza, B. Pratto, M.S.R. dos Santos-Rocha, A.J.G. Cruz, Effect of severity factor on the hydrothermal pretreatment of sugarcane straw, Bioresour. Technol. 275 (2019) 321-327. [92] D. Haldar, D. Sen, K. Gayen, Enzymatic hydrolysis of banana stems (Musa acuminata): Optimization of process parameters and inhibition characterization, Int. J. Green Energy 15(6) (2018) 406-413. [93] H. Lyu, J. Zhou, Z. Geng, C. Lyu, Y. Li, Two-stage processing of liquid hot water pretreatment for recovering C5 and C6 sugars from cassava straw, Process Biochem. 75 (2018) 202-211. [94] R. Biswas, H. Uellendahl, B.K. Ahring, Wet Explosion: a Universal and Efficient Pretreatment Process for Lignocellulosic Biorefineries, Bioenergy Res. 8(3) (2015) 11011116. [95] M. Balat, H. Balat, C. Öz, Progress in bioethanol processing, Prog. Energy Combust. Sci. 34(5) (2008) 551-573. [96] S.S. Toor, L. Rosendahl, A. Rudolf, Hydrothermal liquefaction of biomass: A review of subcritical water technologies, Energy 36(5) (2011) 2328-2342. [97] D. Deswal, R. Gupta, P. Nandal, R.C. Kuhad, Fungal pretreatment improves amenability of lignocellulosic material for its saccharification to sugars, Carbohydr. Polym. 99 (2014) 264-269. [98] J.C. Martínez-Patiño, T.A. Lu-Chau, B. Gullón, E. Ruiz, I. Romero, E. Castro, J.M. Lema, Application of a combined fungal and diluted acid pretreatment on olive tree biomass, Ind. Crops. Prod. 121 (2018) 10-17. [99] A.K. Kumar, S. Sharma, Recent updates on different methods of pretreatment of lignocellulosic feedstocks: a review, Bioresources and bioprocessing 4(1) (2017) 7.
59
Jo
ur
na
lP
re
-p
ro of
[100] R. Ravindran, S. Jaiswal, N. Abu-Ghannam, A.K. Jaiswal, A comparative analysis of pretreatment strategies on the properties and hydrolysis of brewers’ spent grain, Bioresource Technology 248 (2018) 272-279. [101] S.J. Yang, H.Y. Yoo, H.S. Choi, J.H. Lee, C. Park, S.W. Kim, Enhancement of enzymatic digestibility of Miscanthus by electron beam irradiation and chemical combined treatments for bioethanol production, Chemical Engineering Journal 275 (2015) 227-234. [102] P. Kumar, D.M. Barrett, M.J. Delwiche, P. Stroeve, Pulsed Electric Field Pretreatment of Switchgrass and Wood Chip Species for Biofuel Production, Industrial & Engineering Chemistry Research 50(19) (2011) 10996-11001. [103] S.C.T. Oliveira, A.B. Figueiredo, D.V. Evtuguin, J.A. Saraiva, High pressure treatment as a tool for engineering of enzymatic reactions in cellulosic fibres, Bioresource Technology 107 (2012) 530-534. [104] M. Raud, J. Olt, T. Kikas, N2 explosive decompression pretreatment of biomass for lignocellulosic ethanol production, Biomass and Bioenergy 90 (2016) 1-6. [105] R. Ravindran, C. Sarangapani, S. Jaiswal, P.J. Cullen, A.K. Jaiswal, Ferric chloride assisted plasma pretreatment of lignocellulose, Bioresour Technol 243 (2017) 327-334. [106] Q. Qing, C.E. Wyman, Supplementation with xylanase and beta-xylosidase to reduce xylo-oligomer and xylan inhibition of enzymatic hydrolysis of cellulose and pretreated corn stover, Biotechnol Biofuels 4(1) (2011) 18. [107] A.A. Modenbach, S.E. Nokes, Enzymatic hydrolysis of biomass at high-solids loadings – A review, Biomass Bioenergy 56 (2013) 526-544. [108] H. Fang, L. Xia, Cellulase production by recombinant Trichoderma reesei and its application in enzymatic hydrolysis of agricultural residues, Fuel 143 (2015) 211-216. [109] D. Haldar, K. Gayen, D. Sen, Enumeration of monosugars’ inhibition characteristics on the kinetics of enzymatic hydrolysis of cellulose, Process Biochem. 72 (2018) 130-136. [110] Q.-S. Meng, C.-G. Liu, X.-Q. Zhao, F.-W. Bai, Engineering Trichoderma reesei RutC30 with the overexpression of egl1 at the ace1 locus to relieve repression on cellulase production and to adjust the ratio of cellulolytic enzymes for more efficient hydrolysis of lignocellulosic biomass, J. Biotechnol. 285 (2018) 56-63. [111] C.-H. Chen, J.-Y. Yao, B. Yang, H.-L. Lee, S.-F. Yuan, H.-Y. Hsieh, P.-H. Liang, Engineer multi-functional cellulase/xylanase/β-glucosidase with improved efficacy to degrade rice straw, Bioresour. Technol. Rep. 5 (2019) 170-177. [112] Y. Xia, L. Yang, L. Xia, High-level production of a fungal β-glucosidase with application potentials in the cost-effective production of Trichoderma reesei cellulase, Process Biochem. 70 (2018) 55-60. [113] A. Boyce, G. Walsh, Characterisation of a novel thermostable endoglucanase from Alicyclobacillus vulcanalis of potential application in bioethanol production, Appl. Microbiol. Biotechnol. 99(18) (2015) 7515-25. [114] J. Rocha-Martín, C. Martinez-Bernal, Y. Pérez-Cobas, F.M. Reyes-Sosa, B.D. García, Additives enhancing enzymatic hydrolysis of lignocellulosic biomass, Bioresour. Technol. 244 (2017) 48-56. [115] S.D. Mansfield, C. Mooney, J.N. Saddler, Substrate and Enzyme Characteristics that Limit Cellulose Hydrolysis, Biotechnology Progress 15(5) (1999) 804-816. [116] A.R. Esteghlalian, V. Srivastava, N. Gilkes, D.J. Gregg, J.N. Saddler, An Overview of Factors Influencing the Enzymatic Hydrolysis of Lignocellulosic Feedstocks, Glycosyl Hydrolases for Biomass Conversion, American Chemical Society2000, pp. 100-111. [117] X. Pan, D. Xie, N. Gilkes, D.J. Gregg, J.N. Saddler, Strategies to enhance the enzymatic hydrolysis of pretreated softwood with high residual lignin content, Applied biochemistry and biotechnology 124(1) (2005) 1069.
60
Jo
ur
na
lP
re
-p
ro of
[118] A. Asztalos, M. Daniels, A. Sethi, T. Shen, P. Langan, A. Redondo, S. Gnanakaran, A coarse-grained model for synergistic action of multiple enzymes on cellulose, Biotechnol. Biofuels 5(1) (2012) 55. [119] E. Hoshino, M. Shiroishi, Y. Amano, M. Nomura, T. Kanda, Synergistic actions of exo-type cellulases in the hydrolysis of cellulose with different crystallinities, J. Ferment. Bioeng. 84(4) (1997) 300-306. [120] Y. Zhao, B. Wu, B. Yan, P. Gao, Mechanism of cellobiose inhibition in cellulose hydrolysis by cellobiohydrolase, Sci China C Life Sci. 47(1) (2004) 18-24. [121] J. Li, L. Du, L. Wang, Glycosidic-bond hydrolysis mechanism catalyzed by cellulase Cel7A from Trichoderma reesei: A comprehensive theoretical study by performing MD, QM, and QM/MM calculations, The journal of physical chemistry. B 114(46) (2010) 15261-8. [122] V. Prévot, M. Lopez, E. Copinet, F. Duchiron, Comparative performance of commercial and laboratory enzymatic complexes from submerged or solid-state fermentation in lignocellulosic biomass hydrolysis, Bioresource Technology 129 (2013) 690-693. [123] M. Kirupa Sankar, R. Ravikumar, M. Naresh Kumar, U. Sivakumar, Development of co-immobilized tri-enzyme biocatalytic system for one-pot pretreatment of four different perennial lignocellulosic biomass and evaluation of their bioethanol production potential, Bioresource Technology 269 (2018) 227-236. [124] J. Börjesson, M. Engqvist, B. Sipos, F. Tjerneld, Effect of poly(ethylene glycol) on enzymatic hydrolysis and adsorption of cellulase enzymes to pretreated lignocellulose, Enzyme and Microbial Technology 41(1) (2007) 186-195. [125] D. Dahnum, S.O. Tasum, E. Triwahyuni, M. Nurdin, H. Abimanyu, Comparison of SHF and SSF processes using enzyme and dry yeast for optimization of bioethanol production from empty fruit bunch, Energy Procedia 68 (2015) 107-116. [126] W.R.d.S. Trindade, R.G.d. Santos, Review on the characteristics of butanol, its production and use as fuel in internal combustion engines, Renewable Sustainable Energy Rev. 69 (2017) 642-651. [127] K. Charubin, R.K. Bennett, A.G. Fast, E.T. Papoutsakis, Engineering Clostridium organisms as microbial cell-factories: challenges & opportunities, Metab. Eng. 50 (2018) 173-191. [128] P. Majidian, M. Tabatabaei, M. Zeinolabedini, M.P. Naghshbandi, Y. Chisti, Metabolic engineering of microorganisms for biofuel production, Renewable Sustainable Energy Rev. 82 (2018) 3863-3885. [129] N. Singh, A.S. Mathur, R.P. Gupta, C.J. Barrow, D. Tuli, M. Puri, Enhanced cellulosic ethanol production via consolidated bioprocessing by Clostridium thermocellum ATCC 31924☆, Bioresource Technology 250 (2018) 860-867. [130] F. Xin, W. Dong, W. Zhang, J. Ma, M. Jiang, Biobutanol production from crystalline cellulose through consolidated bioprocessing, Trends Biotechnol. 37(2) (2019) 167-180. [131] Y. Jiang, D. Guo, J. Lu, P. Dürre, W. Dong, W. Yan, W. Zhang, J. Ma, M. Jiang, F. Xin, Consolidated bioprocessing of butanol production from xylan by a thermophilic and butanologenic Thermoanaerobacterium sp. M5, Biotechnol. Biofuels 11(1) (2018) 89. [132] S. Brethauer, M.H. Studer, Consolidated bioprocessing of lignocellulose by a microbial consortium, Energy Environ. Sci. 7(4) (2014) 1446-1453. [133] L. Liu, J. Yang, Y. Yang, L. Luo, R. Wang, Y. Zhang, H. Yuan, Consolidated bioprocessing performance of bacterial consortium EMSD5 on hemicellulose for isopropanol production, Bioresource Technology 292 (2019) 121965. [134] L. Liu, Z. Zhang, J. Wang, Y. Fan, W. Shi, X. Liu, Q. Shun, Simultaneous saccharification and co-fermentation of corn stover pretreated by H2O2 oxidative degradation for ethanol production, Energy 168 (2019) 946-952.
61
Jo
ur
na
lP
re
-p
ro of
[135] L.M. Nieves, L.A. Panyon, X. Wang, Engineering Sugar Utilization and Microbial Tolerance toward Lignocellulose Conversion, Frontiers in bioengineering and biotechnology 3 (2015) 17. [136] M.T. Fernández-Sandoval, J. Galíndez-Mayer, F. Bolívar, G. Gosset, O.T. Ramírez, A. Martinez, Xylose–glucose co-fermentation to ethanol by Escherichia coli strain MS04 using single- and two-stage continuous cultures under micro-aerated conditions, Microbial cell factories 18(1) (2019) 145. [137] R. Yao, K. Shimizu, Recent progress in metabolic engineering for the production of biofuels and biochemicals from renewable sources with particular emphasis on catabolite regulation and its modulation, Process Biochemistry 48(9) (2013) 1409-1417. [138] T. Li, J. He, Simultaneous saccharification and fermentation of hemicellulose to butanol by a non-sporulating Clostridium species, Bioresour. Technol. 219 (2016) 430-438. [139] K. Ma, Z. Ruan, Production of a lignocellulolytic enzyme system for simultaneous biodelignification and saccharification of corn stover employing co-culture of fungi, Bioresour. Technol. 175 (2015) 586-93. [140] M. Karimi, R. Esfandiar, D. Biria, Simultaneous delignification and saccharification of rice straw as a lignocellulosic biomass by immobilized Thrichoderma viride sp. to enhance enzymatic sugar production, Renewable Energy 104 (2017) 88-95. [141] Y.-G. Lee, Y.-S. Jin, Y.-L. Cha, J.-H. Seo, Bioethanol production from cellulosic hydrolysates by engineered industrial Saccharomyces cerevisiae, Bioresour. Technol. 228 (2017) 355-361. [142] S.H. Mohd Azhar, R. Abdulla, Bioethanol production from galactose by immobilized wild-type Saccharomyces cerevisiae, Biocatal. Agric. Biotechnol. 14 (2018) 457-465. [143] D.T.T. Nguyen, P. Praveen, K.-C. Loh, Zymomonas mobilis immobilization in polymeric membranes for improved resistance to lignocellulose-derived inhibitors in bioethanol fermentation, Biochem. Eng. J. 140 (2018) 29-37. [144] Q. Zhang, Nurhayati, C.-L. Cheng, Y.-C. Lo, D. Nagarajan, J. Hu, J.-S. Chang, D.-J. Lee, Ethanol production by modified polyvinyl alcohol-immobilized Zymomonas mobilis and in situ membrane distillation under very high gravity condition, Appl. Energy 202 (2017) 1-5. [145] R.M. Ferreira, M.J. Mota, R.P. Lopes, S. Sousa, A.M. Gomes, I. Delgadillo, J.A. Saraiva, Adaptation of Saccharomyces cerevisiae to high pressure (15, 25 and 35 MPa) to enhance the production of bioethanol, Food Res. Int. 115 (2019) 352-359. [146] P.D. Nagy, Exploitation of a surrogate host, Saccharomyces cerevisiae, to identify cellular targets and develop novel antiviral approaches, Curr. Opin. Virol. 26 (2017) 132-140. [147] X.H. Hu, M.H. Wang, T. Tan, J.R. Li, H. Yang, L. Leach, R.M. Zhang, Z.W. Luo, Genetic dissection of ethanol tolerance in the budding yeast Saccharomyces cerevisiae, Genetics 175(3) (2007) 1479-87. [148] J.E. DiCarlo, J.E. Norville, P. Mali, X. Rios, J. Aach, G.M. Church, Genome engineering in Saccharomyces cerevisiae using CRISPR-Cas systems, Nucleic Acids Res. 41(7) (2013) 4336-43. [149] M. Bertilsson, J. Andersson, G. Liden, Modeling simultaneous glucose and xylose uptake in Saccharomyces cerevisiae from kinetics and gene expression of sugar transporters, Bioprocess Biosyst. Eng. 31(4) (2008) 369-77. [150] E. Young, A. Poucher, A. Comer, A. Bailey, H. Alper, Functional survey for heterologous sugar transport proteins, using Saccharomyces cerevisiae as a Host, Appl. Environ. Microbiol. 77(10) (2011) 3311-3319. [151] S. Lane, H. Xu, E.J. Oh, H. Kim, A. Lesmana, D. Jeong, G. Zhang, C.-S. Tsai, Y.-S. Jin, S.R. Kim, Glucose repression can be alleviated by reducing glucose phosphorylation rate in Saccharomyces cerevisiae, Sci. Rep. 8(1) (2018) 2613.
62
Jo
ur
na
lP
re
-p
ro of
[152] A. Farwick, S. Bruder, V. Schadeweg, M. Oreb, E. Boles, Engineering of yeast hexose transporters to transport D-xylose without inhibition by D-glucose, Proceedings of the National Academy of Sciences of the United States of America 111(14) (2014) 5159-5164. [153] A. Ajit, A.Z. Sulaiman, Y. Chisti, Production of bioethanol by Zymomonas mobilis in high-gravity extractive fermentations, Food Bioprod. Process. 102 (2017) 123-135. [154] Y. You, S. Liu, B. Wu, Y.-W. Wang, Q.-L. Zhu, H. Qin, F.-R. Tan, Z.-Y. Ruan, K.-D. Ma, L.-C. Dai, M. Zhang, G.-Q. Hu, M.-X. He, Bio-ethanol production by Zymomonas mobilis using pretreated dairy manure as a carbon and nitrogen source, RSC Adv. 7(7) (2017) 3768-3779. [155] S. Yang, Q. Fei, Y. Zhang, L.M. Contreras, S.M. Utturkar, S.D. Brown, M.E. Himmel, M. Zhang, Zymomonas mobilis as a model system for production of biofuels and biochemicals, Microb. Biotechnol. 9(6) (2016) 699-717. [156] K. Deanda, M. Zhang, C. Eddy, S. Picataggio, Development of an arabinosefermenting Zymomonas mobilis strain by metabolic pathway engineering, Appl. Environ. Microbiol. 62(12) (1996) 4465-70. [157] C. Xue, J. Zhao, L. Chen, S.-T. Yang, F. Bai, Recent advances and state-of-the-art strategies in strain and process engineering for biobutanol production by Clostridium acetobutylicum, Biotechnol. Adv. 35(2) (2017) 310-322. [158] C. Ren, Y. Gu, S. Hu, Y. Wu, P. Wang, Y. Yang, C. Yang, S. Yang, W. Jiang, Identification and inactivation of pleiotropic regulator CcpA to eliminate glucose repression of xylose utilization in Clostridium acetobutylicum, Metab. Eng. 12(5) (2010) 446-54. [159] G. De Bhowmick, A.K. Sarmah, R. Sen, Lignocellulosic biorefinery as a model for sustainable development of biofuels and value added products, Bioresour. Technol. 247 (2018) 1144-1154. [160] J. Song, H. Fan, J. Ma, B. Han, Conversion of glucose and cellulose into value-added products in water and ionic liquids, Green Chem. 15(10) (2013) 2619-2635. [161] S. Zhang, O. Metin, D. Su, S. Sun, Monodisperse AgPd alloy nanoparticles and their superior catalysis for the dehydrogenation of formic acid, Angew. Chem. Int. Ed. Engl. 52(13) (2013) 3681-4. [162] J. Yun, F. Jin, A. Kishita, K. Tohji, H. Enomoto, Formic acid production from carbohydrates biomass by hydrothermal reaction, J. Phys. Conf. Ser. 215 (2010) 012126. [163] W. Wang, M. Niu, Y. Hou, W. Wu, Z. Liu, Q. Liu, S. Ren, K.N. Marsh, Catalytic conversion of biomass-derived carbohydrates to formic acid using molecular oxygen, Green Chem. 16(5) (2014) 2614-2618. [164] P. Gao, G. Li, F. Yang, X.-N. Lv, H. Fan, L. Meng, X.-Q. Yu, Preparation of lactic acid, formic acid and acetic acid from cotton cellulose by the alkaline pre-treatment and hydrothermal degradation, Ind. Crops Prod. 48 (2013) 61-67. [165] F. Jin, Z. Zhou, T. Moriya, H. Kishida, H. Higashijima, H. Enomoto, Controlling hydrothermal reaction pathways to improve acetic acid production from carbohydrate biomass, Environ. Sci. Technol. 39(6) (2005) 1893-1902. [166] F. Jin, Z. Zhou, A. Kishita, H. Enomoto, H. Kishida, T. Moriya, A New hydrothermal process for producing acetic acid from biomass waste, Chem. Eng. Res. Des. 85(2) (2007) 201-206. [167] Z. Huo, Y. Fang, G. Yao, X. Zeng, D. Ren, F. Jin, Improved two-step hydrothermal process for acetic acid production from carbohydrate biomass, J. Energy Chem. 24(2) (2015) 207-212. [168] I. Obregón, I. Gandarias, M.G. Al-Shaal, C. Mevissen, P.L. Arias, R. Palkovits, The role of the hydrogen source on the selective production of γ-Valerolactone and 2Methyltetrahydrofuran from levulinic acid, ChemSusChem 9(17) (2016) 2488-2495.
63
Jo
ur
na
lP
re
-p
ro of
[169] Z. Sun, L. Xue, S. Wang, X. Wang, J. Shi, Single step conversion of cellulose to levulinic acid using temperature-responsive dodeca-aluminotungstic acid catalysts, Green Chem. 18(3) (2016) 742-752. [170] H. Jeong, S.-Y. Park, G.-H. Ryu, J.-H. Choi, J.-H. Kim, W.-S. Choi, S.M. Lee, J.W. Choi, I.-G. Choi, Catalytic conversion of hemicellulosic sugars derived from biomass to levulinic acid, Catal. Commun. 117 (2018) 19-25. [171] A.S. Khan, Z. Man, M.A. Bustam, A. Nasrullah, Z. Ullah, A. Sarwono, F.U. Shah, N. Muhammad, Efficient conversion of lignocellulosic biomass to levulinic acid using acidic ionic liquids, Carbohydr. Polym. 181 (2018) 208-214. [172] H. Kobayashi, Y. Ito, T. Komanoya, Y. Hosaka, P.L. Dhepe, K. Kasai, K. Hara, A. Fukuoka, Synthesis of sugar alcohols by hydrolytic hydrogenation of cellulose over supported metal catalysts, Green Chem. 13(2) (2011) 326-333. [173] R. Palkovits, K. Tajvidi, J. Procelewska, R. Rinaldi, A. Ruppert, Hydrogenolysis of cellulose combining mineral acids and hydrogenation catalysts, Green Chem. 12(6) (2010) 972-978. [174] A. Yamaguchi, O. Sato, N. Mimura, Y. Hirosaki, H. Kobayashi, A. Fukuoka, M. Shirai, Direct production of sugar alcohols from wood chips using supported platinum catalysts in water, Catal. Commun. 54 (2014) 22-26. [175] A. Yamaguchi, O. Sato, N. Mimura, M. Shirai, Catalytic production of sugar alcohols from lignocellulosic biomass, Catal. Today 265 (2016) 199-202. [176] C.V. Nguyen, D. Lewis, W.-H. Chen, H.-W. Huang, Z.A. Alothman, Y. Yamauchi, K.C.W. Wu, Combined treatments for producing 5-hydroxymethylfurfural (HMF) from lignocellulosic biomass, Catal. Today 278 (2016) 344-349. [177] L. Shuai, M.T. Amiri, Y.M. Questell-Santiago, F. Héroguel, Y. Li, H. Kim, R. Meilan, C. Chapple, J. Ralph, J.S. Luterbacher, Formaldehyde stabilization facilitates lignin monomer production during biomass depolymerization, Science 354(6310) (2016) 329-333. [178] W. Wang, M. Wang, J. Huang, X. Zhao, Y. Su, Y. Wang, X. Li, Formate-assisted analytical pyrolysis of kraft lignin to phenols, Bioresour. Technol. (2019). [179] I.S. Goldstein, Reactivity toward phenol of lignin from the hydrolysis of sweetgum wood with concentrated sulfuric acid, J. Wood Chem. Technol. 10(3) (1990) 379-386. [180] H. Kawamoto, Lignin pyrolysis reactions, J. Wood Sci. 63(2) (2017) 117-132. [181] K. Tang, X.-F. Zhou, The degradation of kraft lignin during hydrothermal treatment for phenolics, Pol. J. Chem. Technol. 17(3) (2015) 24. [182] J. Luo, P. Melissa, W. Zhao, Z. Wang, Y. Zhu, Selective lignin oxidation towards vanillin in phenol media, ChemistrySelect 1(15) (2016) 4596-4601. [183] A.W. Pacek, P. Ding, M. Garrett, G. Sheldrake, A.W. Nienow, Catalytic conversion of sodium lignosulfonate to vanillin: Engineering aspects. Part 1. Effects of processing conditions on vanillin yield and selectivity, Ind. Eng. Chem. Res. 52(25) (2013) 8361-8372. [184] M. Fache, B. Boutevin, S. Caillol, Epoxy thermosets from model mixtures of the lignin-to-vanillin process, Green Chem. 18(3) (2016) 712-725. [185] G. Wu, M. Heitz, E. Chornet, The depolymerization of lignin via aqueous alkaline oxidation, in: A.V. Bridgwater (Ed.), Advances in Thermochemical Biomass Conversion, Springer Netherlands, Dordrecht, 1993, pp. 1558-1571. [186] J. Zeng, C.G. Yoo, F. Wang, X. Pan, W. Vermerris, Z. Tong, Biomimetic Fentoncatalyzed lignin depolymerization to high-value aromatics and dicarboxylic acids, ChemSusChem 8(5) (2015) 861-71. [187] P. Azadi, O.R. Inderwildi, R. Farnood, D.A. King, Liquid fuels, hydrogen and chemicals from lignin: A critical review, Renewable Sustainable Energy Rev. 21 (2013) 506523.
64
Jo
ur
na
lP
re
-p
ro of
[188] M. Nagy, K. David, G.J.P. Britovsek, A.J. Ragauskas, Catalytic hydrogenolysis of ethanol organosolv lignin Holzforschung 63(5) (2009) 513-520. [189] J. Kang, S. Irmak, M. Wilkins, Conversion of lignin into renewable carboxylic acid compounds by advanced oxidation processes, Renewable Energy 135 (2019) 951-962. [190] N. Galeotti, F. Jirasek, J. Burger, H. Hasse, Recovery of Furfural and Acetic Acid from Wood Hydrolysates in Biotechnological Downstream Processing, Chemical Engineering & Technology 41(12) (2018) 2331-2336. [191] A. Martinez, M.E. Rodriguez, M.L. Wells, S.W. York, J.F. Preston, L.O. Ingram, Detoxification of dilute acid hydrolysates of lignocellulose with lime, Biotechnol Prog 17(2) (2001) 287-93. [192] G.-T. Jeong, S.-K. Kim, D.-H. Park, Detoxification of hydrolysate by reactiveextraction for generating biofuels, Biotechnology and Bioprocess Engineering 18(1) (2013) 88-93. [193] D.H. Cho, Y.J. Lee, Y. Um, B.I. Sang, Y.H. Kim, Detoxification of model phenolic compounds in lignocellulosic hydrolysates with peroxidase for butanol production from Clostridium beijerinckii, Applied microbiology and biotechnology 83(6) (2009) 1035-43. [194] T. Qu, X. Zhang, X. Gu, L. Han, G. Ji, X. Chen, W. Xiao, Ball milling for biomass fractionation and pretreatment with aqueous hydroxide solutions, ACS Sustainable Chem. Eng. 5(9) (2017) 7733-7742. [195] X. Yuan, S. Liu, G. Feng, Y. Liu, Y. Li, H. Lu, B. Liang, Effects of ball milling on structural changes and hydrolysis of lignocellulosic biomass in liquid hot-water compressed carbon dioxide, Korean J. Chem. Eng. 33(7) (2016) 2134-2141. [196] M.R. Zakaria, S. Fujimoto, S. Hirata, M.A. Hassan, Ball milling pretreatment of oil palm biomass for enhancing enzymatic hydrolysis, Appl. Biochem. Biotechnol. 173(7) (2014) 1778-89. [197] F. Shi, H. Xiang, Y. Li, Combined pretreatment using ozonolysis and ball milling to improve enzymatic saccharification of corn straw, Bioresour. Technol. 179 (2015) 444-451. [198] R.C.d.A. Castro, S.I. Mussatto, I.C. Roberto, A vertical ball mill as a new reactor design for biomass hydrolysis and fermentation process, Renewable Energy 114 (2017) 775780. [199] J.H. Lee, J.H. Kwon, T.H. Kim, W.I. Choi, Impact of planetary ball mills on corn stover characteristics and enzymatic digestibility depending on grinding ball properties, Bioresour. Technol. 241 (2017) 1094-1100. [200] B.-J. Gu, M.P. Wolcott, G.M. Ganjyal, Pretreatment with lower feed moisture and lower extrusion temperatures aids in the increase in the fermentable sugar yields from finemilled Douglas-fir, Bioresour. Technol. 269 (2018) 262-268. [201] H. Liu, B. Pang, Y. Zhao, J. Lu, Y. Han, H. Wang, Comparative study of two different alkali-mechanical pretreatments of corn stover for bioethanol production, Fuel 221 (2018) 2127. [202] B.-J. Gu, G.S. Dhumal, M.P. Wolcott, G.M. Ganjyal, Disruption of lignocellulosic biomass along the length of the screws with different screw elements in a twin-screw extruder, Bioresour. Technol. 275 (2019) 266-271. [203] C. Montiel, O. Hernández-Meléndez, E. Vivaldo-Lima, M. Hernández-Luna, E. Bárzana, Enhanced bioethanol production from blue agave bagasse in a combined extrusion– saccharification process, Bioenergy Res. 9(4) (2016) 1005-1014. [204] W.I. Choi, H.J. Ryu, S.J. Kim, K.K. Oh, Thermo-mechanical fractionation of yellow poplar sawdust with a low reaction severity using continuous twin screw-driven reactor for high hemicellulosic sugar recovery, Bioresour. Technol. 241 (2017) 63-69.
65
Jo
ur
na
lP
re
-p
ro of
[205] Y. Liu, L. Guo, L. Wang, W. Zhan, H. Zhou, Irradiation pretreatment facilitates the achievement of high total sugars concentration from lignocellulose biomass, Bioresour. Technol. 232 (2017) 270-277. [206] M. Molaverdi, K. Karimi, S. Mirmohamadsadeghi, Improvement of dry simultaneous saccharification and fermentation of rice straw to high concentration ethanol by sodium carbonate pretreatment, Energy 167 (2019) 654-660. [207] A. Sawisit, S. Jampatesh, S.S. Jantama, K. Jantama, Optimization of sodium hydroxide pretreatment and enzyme loading for efficient hydrolysis of rice straw to improve succinate production by metabolically engineered Escherichia coli KJ122 under simultaneous saccharification and fermentation, Bioresour. Technol. 260 (2018) 348-356. [208] X. Qi, L. Yan, F. Shen, M. Qiu, Mechanochemical-assisted hydrolysis of pretreated rice straw into glucose and xylose in water by weakly acidic solid catalyst, Bioresour. Technol. 273 (2019) 687-691. [209] K.-L. Chang, X.-M. Chen, Y.-J. Han, X.-Q. Wang, L. Potprommanee, X.-a. Ning, J.-y. Liu, J. Sun, Y.-P. Peng, S.-y. Sun, Y.-C. Lin, Synergistic effects of surfactant-assisted ionic liquid pretreatment rice straw, Bioresour. Technol. 214 (2016) 371-375. [210] H. Amiri, K. Karimi, H. Zilouei, Organosolv pretreatment of rice straw for efficient acetone, butanol, and ethanol production, Bioresour. Technol. 152 (2014) 450-456. [211] X. Xie, X. Feng, S. Chi, Y. Zhang, G. Yu, C. Liu, Z. Li, B. Li, H. Peng, A sustainable and effective potassium hydroxide pretreatment of wheat straw for the production of fermentable sugars, Bioresour. Technol. Rep. 3 (2018) 169-176. [212] Z. Yuan, G. Li, E.L. Hegg, Enhancement of sugar recovery and ethanol production from wheat straw through alkaline pre-extraction followed by steam pretreatment, Bioresour. Technol. 266 (2018) 194-202. [213] Z. Zhao, X. Chen, M.F. Ali, A.A. Abdeltawab, S.M. Yakout, G. Yu, Pretreatment of wheat straw using basic ethanolamine-based deep eutectic solvents for improving enzymatic hydrolysis, Bioresour. Technol. 263 (2018) 325-333. [214] X. Chen, H. Li, S. Sun, X. Cao, R. Sun, Co-production of oligosaccharides and fermentable sugar from wheat straw by hydrothermal pretreatment combined with alkaline ethanol extraction, Ind. Crops Prod. 111 (2018) 78-85. [215] H. Zhang, G. Lyu, A. Zhang, X. Li, J. Xie, Effects of ferric chloride pretreatment and surfactants on the sugar production from sugarcane bagasse, Bioresour. Technol. 265 (2018) 93-101. [216] Z. Zhu, C.A. Rezende, R. Simister, S.J. McQueen-Mason, D.J. Macquarrie, I. Polikarpov, L.D. Gomez, Efficient sugar production from sugarcane bagasse by microwave assisted acid and alkali pretreatment, Biomass Bioenergy 93 (2016) 269-278. [217] P.F. Avila, M.B.S. Forte, R. Goldbeck, Evaluation of the chemical composition of a mixture of sugarcane bagasse and straw after different pretreatments and their effects on commercial enzyme combinations for the production of fermentable sugars, Biomass Bioenergy 116 (2018) 180-188. [218] M. Espirito Santo, C.A. Rezende, O.D. Bernardinelli, N. Pereira, A.A.S. Curvelo, E.R. deAzevedo, F.E.G. Guimarães, I. Polikarpov, Structural and compositional changes in sugarcane bagasse subjected to hydrothermal and organosolv pretreatments and their impacts on enzymatic hydrolysis, Ind. Crops Prod. 113 (2018) 64-74. [219] H. Yu, Y. You, F. Lei, Z. Liu, W. Zhang, J. Jiang, Comparative study of alkaline hydrogen peroxide and organosolv pretreatments of sugarcane bagasse to improve the overall sugar yield, Bioresour. Technol. 187 (2015) 161-166. [220] J. Li, W. Li, M. Zhang, D. Wang, Boosting the fermentable sugar yield and concentration of corn stover by magnesium oxide pretreatment for ethanol production, Bioresour. Technol. 269 (2018) 400-407. 66
Jo
ur
na
lP
re
-p
ro of
[221] L. Qin, X. Li, J.-Q. Zhu, W.-C. Li, H. Xu, Q.-M. Guan, M.-T. Zhang, B.-Z. Li, Y.-J. Yuan, Optimization of ethylenediamine pretreatment and enzymatic hydrolysis to produce fermentable sugars from corn stover, Ind. Crops Prod. 102 (2017) 51-57. [222] Q. Qing, L. Zhou, Q. Guo, X. Gao, Y. Zhang, Y. He, Y. Zhang, Mild alkaline presoaking and organosolv pretreatment of corn stover and their impacts on corn stover composition, structure, and digestibility, Bioresour. Technol. 233 (2017) 284-290. [223] H. Teramura, K. Sasaki, T. Oshima, H. Kawaguchi, C. Ogino, T. Sazuka, A. Kondo, Effective usage of sorghum bagasse: Optimization of organosolv pretreatment using 25% 1butanol and subsequent nanofiltration membrane separation, Bioresour. Technol. 252 (2018) 157-164. [224] Y. Jafari, H. Amiri, K. Karimi, Acetone pretreatment for improvement of acetone, butanol, and ethanol production from sweet sorghum bagasse, Appl. Energy 168 (2016) 216225. [225] R. Choudhary, A.L. Umagiliyage, Y. Liang, T. Siddaramu, J. Haddock, G. Markevicius, Microwave pretreatment for enzymatic saccharification of sweet sorghum bagasse, Biomass Bioenergy 39 (2012) 218-226. [226] J. Hu, V. Arantes, J.N. Saddler, The enhancement of enzymatic hydrolysis of lignocellulosic substrates by the addition of accessory enzymes such as xylanase: is it an additive or synergistic effect?, Biotechnol. Biofuels 4(1) (2011) 36. [227] H. Inoue, S.R. Decker, L.E. Taylor, S. Yano, S. Sawayama, Identification and characterization of core cellulolytic enzymes from Talaromyces cellulolyticus (formerly Acremonium cellulolyticus) critical for hydrolysis of lignocellulosic biomass, Biotechnol. Biofuels 7(1) (2014) 151. [228] Y. Yang, J. Yang, J. Liu, R. Wang, L. Liu, F. Wang, H. Yuan, The composition of accessory enzymes of Penicillium chrysogenum P33 revealed by secretome and synergistic effects with commercial cellulase on lignocellulose hydrolysis, Bioresour. Technol. 257 (2018) 54-61. [229] R. Kumar, C.E. Wyman, Effects of cellulase and xylanase enzymes on the deconstruction of solids from pretreatment of poplar by leading technologies, Biotechnol. Progr. 25(2) (2009) 302-314. [230] K. Murashima, A. Kosugi, R.H. Doi, Synergistic effects of cellulosomal xylanase and cellulases from Clostridium cellulovorans on plant cell wall degradation, J. Bacteriol. 185(5) (2003) 1518-1524. [231] A. Duque, P. Manzanares, M. Ballesteros, Extrusion as a pretreatment for lignocellulosic biomass: Fundamentals and applications, Renewable Energy 114 (2017) 1427-1441. [232] R. Ma, M. Guo, X. Zhang, Selective conversion of biorefinery lignin into dicarboxylic acids, ChemSusChem 7(2) (2014) 412-415.
67
Table 1 Major ongoing R&D projects on biofuels in India.
2.
R&D Institute Enhanced butanol production NEERI, from lignocellulosic biomass Nagpur using improved pretreatment and integrated saccharification, fermentation and separation in a membrane bioreactor Development of pretreatment DU, New strategies and bioprocess for Delhi improved production of cellulolytic enzymes and ethanol from crop byproduct for demonstration at pilot plant
Sanctioned Year 2011
Sanctioned amount INR (USD) 39.7 lakhs (55,810)
2012
148.5 lakhs (2,08,761)
ro of
1.
Project Title
-p
Sl. No.
Process development for bioethanol production from agricultural residues PhaseI: Development of process for co-fermentation of hexose and pentose sugars of agricultural residues
SSSNRE, Punjab
4.
Hydropyrolysis of lignocellulosic biomass to value-added hydrocarbons
IIP, Dehradun
2012
186.4 lakhs (2,62,144)
5.
Sorghum stover based biorefinery for fuels and chemicals
NIIST, Trivandrum; MNNIT, Allahabad; TERI, New Delhi & IICT, Hyderabad JCE, Mysore
2012
184.14 lakhs (2,58,965)
2012
24 lakhs (33,742)
lP
na
ur
Jo 6.
Isolation and characterization of Diatom species and process development for biofuel production
2012
132.2 lakhs (1,85,919)
re
3.
68
Engineering Escherichia coli strains optimized for large scale lignocelluloses fermentation for biofuel production
IIT, Bombay
2012
25 lakhs (35,148)
8.
Stabilization and up gradation of biomass derived bio-oils over tailored multifunctional catalysts in a dual stage catalytic process to produce liquid hydrocarbon fuels and its application studies
TERI, New Delhi
2013
164 lakhs (2,30,568)
9.
Improved production of biogas and bio-CNG from lignocellulosic biomass
10.
Development of new catalytic systems for efficient transformation of biomass/biomass derivatives to biofuel components
DBT-ICT 2013 Centre for Energy Biosciences, ICT, Matunga (E), Mumbai; Kirloskar Integrated Technologies Limited, Pune; Glycols Limited, Noida IIT, Indore 2017
ro of
7.
Jo
ur
na
lP
re
-p
445.90 lakhs (6,26,779)
69
54.15 lakhs (76,119)
Table 2 Effect of ball milling on pretreatment of biomass. Sl. No.
Biomass
Pretreatment method
Major effects
Ref
1.
Wheat straw
Ball milling with alkali hydrolysis
[194]
2.
Camphorwood Ball milling with saw dust carbon-di-oxide catalyzed hydrothermal treatment
3.
Oil palm fruit fiber
Ball milling
4.
Corn straw
Ball milling
-Ultrafine structure of biomass was formed -Removal of hemicellulose and lignin was 93.8% and 86.1% respectively -A maximum glucose of 98.5% was obtained after 72 h of enzymatic hydrolysis -Cellulose crystallinity was decreased to 21.4% from 60.9% of raw biomass -Conversion of cellulose was 37.8% -A maximum glucose of 1.5 g/L was obtained in the hydrolysate -After 60 min of milling at 250 rpm, 80.3% glucose and 78.6% xylose was obtained from enzymatic hydrolysis - After 60 min of milling at 250 rpm, 89.6% reduction in size of the biomass attributes to 4.5% of crystallinity index (CI) compared to 57% of CI for untreated biomass -86.4% reduction in average particle size of the biomass when it was milled at 1332 rpm for 12 min while specific surface area was increased by 1143% compared to 26.68 m2/kg for untreated biomass -48% reduction in crystallinity was achieved under the above condition -26.5% of glucose and 10% of xylose was produced after saccharification of biomass milled for 8 min -A maximum cellulose conversion of 87% was observed from 24 h of enzymatic hydrolysis with the biomass milled at 100 rpm using 30 unit of glass sphere in the reaction system
ro of
[195]
Jo
ur
na
lP
re
-p
[196]
5.
Rice straw
Ball milling using vertical reactor
70
[197]
[198]
Corn stover
[199]
Jo
ur
na
lP
re
-p
ro of
6.
-A maximum of 21.7 g/L ethanol was produced with 100% of glucose uptake during fermentation of ball mill treated biomass Planetary ball milling -Steel ball produced 73% of using steel, zicornia particle (<100 µm) when biomass and alumina ball was milled at 300 rpm for 20 min while alumina and zicornia produced only 52% of biomass particle -Compared to steel and zicornia, biomass milled for 60 min with alumina ball leads to produce a maximum of 92% of glucose after enzymatic hydrolysis
71
Table 3 Effect of extrusion pretreatment on lignocellulosic biomass. Variables -Temperature (25, 50, 100, 150 °C) -Moisture (30, 40, 50 % (w/w))
2.
Corn stover
-Reaction period of enzymatic hydrolysis
3.
Doulas fir
4.
Agave bagasse
-Maximum glucose of 39.3% [202] was obtained after enzymatic hydrolysis of extruded biomass -Lowest screw speed of 25 rpm produced maximum sugar yield of ~37% -Reverse screw element significantly improved sugar yield after enzymatic hydrolysis of biomass -Enzyme - A maximum of ~85% [203] dosage at hydrolysis yield was obtained different when 5% (w/v) extruded consortium biomass was treated with -Variation in Celic CT2 enzyme for72 h the -In the presence of Celic CT2 concentration enzyme, 20% (w/w) biomass of pretreated showed 73% of hydrolysis biomass yield with a production of around 70 g/L glucose
re
-Temperature of barrel (50, 100, 150 °C) -Screw speed (25, 50, 75 rpm)
na
lP
Extrusion followed by chemical pretreatment with NaOH at 0.06 g NaOH/g biomass at liquid to solid loading of 2:1 Extrusion using twin screw extruder with length to diameter of 40.5 for biomass contained 50% of moisture content
Jo
ur
Extrusion using twin screw extruder constructed with 10 different modules specific for the subsequent functions of feeding, NaOH treatment, neutralization, filtration and compression
Major effect of pretreatment Ref -Maximum sugars of ~42% [200] was obtained at lowest moisture content of 30% and lowest temperature of 25 °C -Compared to the other decrement, a significant decrement in the crystallinity was observed at 25 °C temperature and biomass of 40% moisture content -Delignification of biomass [201] was 69.5% -Total sugar yield was 78.8% after 24 h of enzymatic hydrolysis
ro of
Biomass Pretreatment Douglas Extrusion using fir twin screw extruder with 24 screw elements and 1 reverse element at speed of 25 rpm
-p
Sl. No. 1.
72
-Hemicellulosic sugars (xylose, mannose and galactose) showed a maximum of 74.8% yield at 127 °C and with an addition of 0.8% (w/w) acid concentration -Double fold increase in surface was observed for extruded sample compared to untreated biomass (0.34 m2/g)
[204]
ro of
-Temperature of barrel (115 -160 °C) -Acid concentration (0.5 – 1.4% (w/w))
Jo
ur
na
lP
re
-p
5.
using two reverse screws rotated at 100 rpm Yellow Continuous poplar twin screw saw dust driven reactor was constituted of barrel. Shaft, screw, feeding system and drive motor with length/diameter of the screw in the reaction zone was 34.4 and rotation was fixed at 25 rpm
73
Table 4 Assessment of the effect of irradiation pretreatment on the various parameters for different biomass.
Sugarcane bagasse
Pretreatment method Gamma radiation
Electron beam radiation with acid hydrolysis
Lignin content
Kenaf core
Electron beam radiation with acid hydrolysis
Crystallinity index
Sugarcane bagasse
Effect of pretreatment Compared to 45.4% (w/w) in the raw biomass, cellulose content decreased to 11.2% (w/w) at 2000 kGy of gamma radiation 500 kGy of Compared to 25.4% electron beam (w/w) present in dose with 3% raw biomass, (v/v) sulphuric hemicellulose acid content of the pretreated biomass was observed as 1.4% (w/w) -50 kGy of Sole pretreatment electron beam with electron beam -Acid hydrolysis or acid dosing with 3% (v/v) didn’t produce any sulphuric acid significant changes -500 kGy of in lignin content of electron beam pretreated biomass. dose with 3% However, at high (v/v) sulphuric dosing of electron acid beam with sequential acid hydrolysis resulted in 45.1% reduction in lignin content More than 100 Compared to raw kGy of gamma biomass 30% radiation reduction in crystallinity index was observed at 500 kGy of gamma radiation 800 kGy of Following gamma radiation irradiation, and subsequent consumption of milling energy during milling was around 120 kWh/ton of oven dried biomass
Ref [59]
[57]
[57]
Jo
ur
na
lP
re
Hemicellulose Kenaf core content
Reaction condition More than 100 kGy of gamma radiation
ro of
Biomass
-p
Parameter studied Cellulose content
Milling energy consumption
Gamma radiation
Cedarwood Gamma radiation
74
[59]
[205]
while for unirradiated biomass it was more than 400 kWh/ton of oven dried biomass
Pine
Gamma radiation
800 kGy of gamma radiation and subsequent milling
After irradiation, following milling for 1 min, less than 180 µm particle size of around 55.9% (w/w) was obtained compared to 35.5% (w/w) of untreated biomass milled for 4 min
Jo
ur
na
Eucalyptus
lP
re
-p
Particle size
Following irradiation, consumption of energy during milling was around 100 kWh/ton of oven dried biomass while for unirradiated biomass it was more than 400 kWh/ton of oven dried biomass
ro of
Pine
75
After irradiation, following milling for 1 min, less than 180 µm particle size of around 46.7% (w/w) was obtained compared to 37.9% (w/w) of untreated biomass milled for 4 min
[205]
Table 5 Effect of different physicochemical pretreatment methods on the saccharification of lignocellulosic residues. Remarks on Saccharification -Conversion of 91% and highest glucose yield of 124.7 g/L
Ref
1.
Rice straw
-0.5 M Na2CO3 at 93 °C for 5 h -2.0 M NaOH at 121 °C for 15 min
-After 5 days of enzymatic hydrolysis, 63.4% (w/w) total sugars was obtained
[207]
-6% (w/w) KOH treated 0.1 g biomass with 0.05 g C (generated from pretreated hydrolysate) ball milled for 2 h and subsequent hydrolysis with water at 200 °C for 60 min
-19.4% (w/w) glucose [208] and 61.5% (w/w) xylose was obtained
-After 72 h of enzymatic hydrolysis, 5.2 g/L of reducing sugar was obtained with cellulose conversion of ~35%
-75% (v/v) aqueous ethanol with 1% (w/w) sulphuric acid at 180 °C for 30 min -15% KOH, 120 °C for 40 min
-15.1 g/L glucose, 5.6 [210] g/L xylose and 1 g/L arabinose was obtained after 72 h of enzymatic hydrolysis -80% yield in total [211] sugars with ~90 % and ~70% yield of glucose and xylose respectively at 12 h of enzymatic hydrolysis
-Sequential treatment with 8% (w/w) NaOH at 80° C for 90 min followed by steam explosion with 3% (w/w) SO2 at 151 °C for
-80% (w/w) glucose and 65% (w/w) xylose was obtained at 25% (w/v) of solid loading during enzymatic hydrolysis
ur
na
lP
-Biomass was pretreated with 5 g ([BMIM]Cl) as ionic liquid at 110 °C for 60 min in the presence of SDS as a surfactant
Wheat straw
Jo
2.
[206]
ro of
Pretreatment
-p
Biomass
re
Sl. No.
76
[209]
[212]
16 min [213]
-Hydrothermal pretreatment with water at 180 °C for 30 min
-Pretreated filtrate was obtained with 6 g and 60 g of xylose and xylooligosaccharides respectively per kg of biomass and enzymatic hydrolysis of pretreated biomass reported 55% of glucan conversion 80.1% of glucose yield was observed after 72 h of enzymatic hydrolysis
[214]
-86% sugar yield was observed in which 64% was glucose
[216]
-[EMIM]Ac as an ionic -99.5% and 86.5% of liquid at 25 °C for 30 glucose and xylose min in an ultrasonic bath was reported after enzymatic hydrolysis
[217]
-Hot water pretreatment with 0.025 M FeCl3 at 160 °C for 10 min
[215]
na
lP
re
-Microwave assisted heating for 7 min in the presence of 0.2 M H2SO4
ro of
Sugarcane bagasse
-90% of cellulose and 62% of xylan conversion in enzymatic hydrolysis
-p
3.
-Choline chloride: monoethanolamine as deep eutectic solvent, 70 °C for 9 h
-76.8% enzymatic hydrolysis yield was obtained based on cellulose conversion into glucose
-Green liquid in combinations with ethanol-water [50:50% (v/v)] at 140 °C for 4 h
-Glucose and xylose [219] yield of 97.7% and 94.1% respectively was obtained after 72 h of enzymatic hydrolysis -71.5% (w/w) glucose [65] yield was reported after 72 h of enzymatic hydrolysis
Jo
ur
-Water as hydrothermal media at 160 °C for 30 min followed by 50% (v/v) ethanol-water as organosolv media at 190 °C for 100 min
4.
Corn stover
-Dilute acid in 1% (w/w) HCl at 140 °C for 40 min in combination with ammonia wet 77
[218]
oxidation under the condition 12% (w/w) NH4OH, 3 MPa O2 at 130 °C 40 min
-2 g/g solid NH3 loading at 60 °C for 2 d in coupled with ethylenediamine loading of 0.6 mL/ g of biomass at 100 °C for 20 min
-28.8 g/L glucose and 11 g/L of xylose was obtained after 72 h of enzymatic hydrolysis
[221]
-Presoaked in 1% (w/w) Na2S at 40 °C for 4 h followed by organosolv pretreatment with 0.2% (w/v) HCl in 20% methanol at 160 °C for 20 min -25% (v/v) 1-butanol in 0.5% (w/w) H2SO4 at 200 °C for 60 min
-More than 80% yield in total sugars were reported after 72 h of enzymatic hydrolysis.
-p
ro of
[220]
[222]
-About 95% of glucose yield was obtained after 72 h of enzymatic hydrolysis
[223]
-50% (v/v) acetonewater with 0.1% (w/w) H2SO4 at 180 °C for 60 min
-78 g/L glucose and 12 g/L xylose was obtained after 72 h of enzymatic hydrolysis
[224]
-Microwave treatment with 10 mL/ g of biomass for 4 min
-40% (w/w) total reducing sugars was reported after 72 h of enzymatic hydrolysis
[225]
re
Sorghum bagasse
-Total sugar of 50 g/L was obtained after 120 h of enzymatic hydrolysis
Jo
ur
na
lP
5.
-0.1 mol/L MgO at 190 °C for 40 min
78
Table 6 Effect of mixed consortium of enzymes on the saccharification of biomass. Enzyme cocktail
Lignocellulose
Effect on hydrolysis 87%a
Ref
1.
Cellulase (Celluclast 1.5 L) + multifect xylanase
Steam pretreated corn stover (SPCS)
2.
Multifect xylanase + cellulase (Celluclast 1.5 L)
SPCS
100% a1
[226]
3.
Core enzyme mixture (Cel7A, Cel6A, Cel5A, Xyl10A, Bgl3A)
Pretreated corn stover
80% b
[227]
4.
Cellulase (T. longibrachiatum) + P33 enzyme cocktail (P. chrysogenum)
Corn stover
5.
Spezyme CP cellulase + Multifect xylanase
Dilute acid pretreated poplar solid
58.2%e,f
[229]
6.
Xylanase XynA + cellulase
Corn cell wall
0.622 µmol/ming
[230]
lP
re
-p
83%c , 44%d
[228]
Cellulose of SPCS converted after 72 h of enzymatic hydrolysis; a1 Xylan of SPCS converted after 72h of enzymatic hydrolysis; b After 48 h of enzymatic hydrolysis; c Xylan conversion after 120 h of enzymatic hydrolysis; d Glucan conversion after 120 h of enzymatic hydrolysis; e After 24 h of hydrolysis; f Glucose released; g After 15 h of enzymatic reaction
Jo
ur
na
a
[226]
ro of
Sl. No.
79
Table 7 List of companies involved in the production of biofuel from biomass. Country
Biomass used
1.
Poet Dsm (http://www.poetdsm.com, 12 March 2019)
USA
Corn crop
2.
Abengoa bioenergy (http://www.abengoa.com, 12 March 2019)
Spain & USA
3.
Clariant (http://www.clariant.com, 12 March 2019)
Switzerland
wheat, barley, corn, sorghum, corn stover, wheat straw, oat straw, barley straw, hardwood, switchgrass wheat straw, rice straw, corn stover and sugar cane bagasse
4.
Raizen-Iogen Brazil (https://www.raizen.com.br) (http://www.iogen.ca, 12 March 2019)
5.
Ethtec Australia (https://www.ethtec.com.au, 12 March 2019)
-p re
lP na
ur
Jo
80
Recent breakthrough Novel approach in the pretreatment of corn feedstock excels the production of cellulosic ethanol Innovative enzymatic process manifests 25 million gallon of ethanol per year
ro of
Sl. No. Name of the company
Sugarcane bagasse and straw
Crop stubbles, cotton gin trash, timber residues, sugarcane bagasse
Recent partnership with Exxonmobil and renewable energy group jointly venture on integrated pretreatment process towards cost effective biodiesel production The plant located at Piracicaba successfully installed 42 million liters of production unit of bioethanol Fund disbursement of $11.9 million dollar by Australian renewable energy agency to Ethtec enable the company to build the latest pilot lignocellulosic
fuel plant in the Hunter valley region Gevo USA (https://www.gevo.com) (http://www.renmatix.com, 12 March 2019)
Wood, agricultural residues
Renmatix plantrose process in a joint venture with Geno’s GIFT technology was aimed to evaluate the feasibility of producing jet fuel alcohol
Jo
ur
na
lP
re
-p
ro of
6.
81
Table 8 Comparison among various fermentation methods available for lignocellulosic biomass.
ur
Simultaneous pretreatment and saccharification
Jo
3.
4.
Consolidated bioprocessing
-Pretreatment and saccharification of biomass is carried out together followed by fermentation -Pretreatment, saccharification and fermentation process of 82
ro of
Demerits -Product inhibition due to prolonged hydrolysis -Cost intensive process - Overall time consuming process -Chances of contamination is high
-p
Simultaneous saccharification and fermentation
Merits -The optimum conditions for hydrolysis and fermentation is appropriately maintained as both of the reactions are carried out in separate reactors
re
2.
Features -After pretreatment, biomass is hydrolyzed at optimum reaction conditions favorable for enzyme -The hydrolysate obtained after enzymatic hydrolysis is utilized as feedstock for fermentation at reaction conditions favorable for fermentation -After an initial pretreatment process, biomass is hydrolyzed and fermented simultaneously
lP
Method Separate hydrolysis and fermentation
na
Sl. No. 1.
-Chances of product inhibition is less -Process cost is less as no separate reactor is required for subsequent hydrolysis and fermentation process -Less complex process -Process cost is low -Chances of sugars loss is low
-Difficult to maintain the favorable reactions condition as the temperature required for enzymatic hydrolysis is 50 °C while for fermentation 37 °C has been an optimum
-No complexity involved in the process -Economically most favorable
-Different optimum conditions for different reactions involved during the bioprocess.
-Difficult to maintain different optimum conditions both for pretreatment and saccharification
biomass is carried out in a single reactor with the presence of microbes.
Solid state fermentation
-Scale up of this process is limited at bench scale level -Problem with heat and mass transfer
Jo
ur
na
lP
re
-p
ro of
5.
as extraneous addition of enzyme is not required thus save the maximum cost of the overall process -The process -Higher yield facilitates the -Efficient growth of the utilization of microorganisms biomass as sole on solid carbon source materials at low -Improved moisture product content characteristics
83
ro of -p re
Jo
ur
na
lP
Fig. 1. Pictorial representation of the overall process involved in the production in biofuel from biomass.
84
Amount sanctioned in INR
300 2015-16
250
2016-17
2017-18
200 150 100 50 0 MOP
DST
CSIR
MNRE
MOPNG
DBT
ro of
Government agencies
Jo
ur
na
lP
re
-p
Fig. 2. Budgetary allocations implemented through different R&D agencies of Government of India in the recent years. [MOP: Ministry of Power; DST: Department of Science & Technology; CSIR: Council of Scientific & Industrial Research; MNRE: Ministry of New & Renewable Energy; MOPNG: Ministry of Petroleum and Natural Gas; DBT: Department of Biotechnology] (Source: Mission Innovation: India-Country Report and Progress Update, May 2018).
85
ro of
Jo
ur
na
lP
re
-p
Fig. 3. Design of a typical twin screw extruder. A) Motor; B) Hopper; C) Thermal regulated barrels; D) Screws; E) Pumps. Reprinted with permission from [231].
86
ro of -p re lP
Jo
ur
na
Fig. 4. Schematic representation of enzymatic hydrolysis of biomass.
87
Jo
ur
na
lP
re
-p
ro of
Fig. 5. Proposed mechanism of acetic acid formation from carbohydrate [167].
88
ro of -p re lP
Jo
ur
na
Fig. 6. Lignin depolymerization and aromatic nuclei oxidation (A); Aromatic ring cleavage (B); Formation of carboxylic acids (C). Reprinted with permission from John Wiley and Sons [232].
89